This guide will help you if you haven't played Terraria on expert before, but don't feel like you *have* to follow it verbatim.
I mostly wrote it as a list of reminders for myself.

Step 1:
 * Get a little wood (~100) and make a basic shelter to survive your first night. For now, all you need is an ugly wooden rectangle to protect you. For now I'd recommend against background walls to save time, and if you're spending more than 2 minutes building it, you're getting too involved. Wait until a little later in the game when you have more materials and faster tools to work with. If you really want to expand your shelter ASAP, then do it during your first night when you're stuck inside due to all the enemies outside. Make the most out of your daylight in the early game.
 * Get 85 cacti to make basic armor and a better sword. While in the desert, be prepared to place dirt, wood, or stone blocks to create walls to protect yourself from deadly enemies, especially antlion chargers, which move fast as hell and can kill you in 3 hits. If there's a sandstorm, don't even bother with the desert for now and move on to something else. Sandstorms drastically affect your movement speed and spawn even faster, deadlier enemies that can kill you in 2 hits.
http://terraria.gamepedia.com/Cactus_armor
http://terraria.gamepedia.com/Cactus_Sword
 * Make an iron bow or better, craft at least 1,000 wooden arrows, then craft some of them into flaming arrows. 1 wood and 1 stone are all you need to make 25 wooden arrows, so a meager 40 wood and 40 stone are enough to craft 1,000 wooden arrows. You'll need 1 gel, 1 wood, and 30 wooden arrows for every 30 flaming arrows you want to craft.
http://terraria.gamepedia.com/Shadewood_Bow
http://terraria.gamepedia.com/Wooden_Arrow
http://terraria.gamepedia.com/Flaming_Arrow

 * Get a lot of wood (500-999). This doesn't take as long as it sounds, especially if you start off by only getting 100-300 wood, then come back to this task after you've obtained a silver axe or better.
 * Once you have at least 250 wood and 250 stone, or at least 500 wood and 0 stone, build homes for NPCs. Building the NPC homes about 7-10 blocks above the ground keeps them safe from most enemies, aside from bosses. I prefer to make my NPC hotel/prison a short distance away from the shelter I spawn in, rather than having us all live inside the same building, and later on I build an event arena on the opposite side of my shelter, about 75-150 blocks away. That way, goblin invasions, martian invasions, etc. have virtually no chance of slaughtering friendly NPCs since they're too far away for enemies to spawn near them, so long as I stay near the event arena. http://terraria.gamepedia.com/NPC
 * Get some cobwebs (20 + 30 more per string) and craft them into 1 yoyo and 3+ strings to gain 1-4 defense. When you craft an accessory, a weapon, or a tool, it'll probably have a random modifier that increases or decreases its stats a little. A +1-4 defense buff on a string is helpful in the early game. You can't equip 2 of the same accessory, so the only point in crafting multiple strings is the chance of getting a better modifier than the last one you got. Yoyos are also helpful early on because they let you attack enemies from a distance without getting hit, and don't require ammo. White strings roughly double the range of yoyos, making this advantage even stronger. If you ever craft an item with a negative modifier, like a Broken/Damaged/Shoddy/Weak yoyo, you should sell it and craft a new one if you have the materials to do so.
http://terraria.gamepedia.com/Wooden_Yoyo
http://terraria.gamepedia.com/String
http://terraria.gamepedia.com/Accessory
http://terraria.gamepedia.com/Modifier

 * Get 48 gold or platinum ore to make a gold/platinum pickaxe. As long as you have the gear listed above (cactus armor, iron+ bow, wooden yoyo without a negative modifier, optional string), then dig straight down to the cavern layer without wasting too much time on the underground layer. The cavern layer has more deposits of gold and platinum ore.
http://terraria.gamepedia.com/Cavern
 * Get 28 gold or platinum ore to make a gold/platinum bow. Go to the snow biome afterward, mine some ice, and craft 900 or more frostburn arrows. You'll need 1 ice block, 1 gel, 1 wood, and 30 wooden arrows for every 30 frostburn arrows you want to craft. That means you'll only need 30 ice blocks, 30 gel, 66 wood, and 36 stone (accounting for the costs of the torches, crafting those into ice torches, and the wooden arrows) to craft 900 frostburn arrows.
http://terraria.gamepedia.com/Platinum_Bow
http://terraria.gamepedia.com/Frostburn_Arrow

Step 2a:
 * Choose whether you want to follow step 2a or 2b. Following both is unnecessary.
 * Go underground in the middle 1/3rd of the map and explore the cavern layer. (The middle 1/3rd of the map is the safest. It's also where you respawn, so if you die, you won't have to walk as far to get back to where you were.) Optionally, you can start digging a hellevator while you're at it. A hellevator is a vertical mineshaft that connects the surface to hell/the underworld. The main goal here is to find 2-4 life crystals to get 140-180 HP. You should avoid getting 200 HP until after you've made an arena for the eye of cthulhu, because once you have 200 HP, it can spawn at night whether you want it to or not. If you're unprepared and don't even have an arena for it, let alone proper armor and weapons, you'll almost certainly waste time dying to it.
 * Another one of your primary goals is to find bombs by breaking pots at the cavern layer or lower. The demolitionist NPC can only move in after you obtain bombs, and obtaining them randomly from pots in the cavern layer or lower is the fastest way to get them.
 * Dig up the first pressure plate or dart trap you find and keep it on your hotbar at all times when exploring underground. Swap to it occasionally to reveal hidden wires, which reveals where traps are.
http://terraria.gamepedia.com/Guide:Travel#Hellevator
http://terraria.gamepedia.com/Crystal_Heart
http://terraria.gamepedia.com/Eye_of_Cthulhu

Step 2b:
 * Choose whether you want to follow step 2a or 2b. Following both is unnecessary.
 * Go to the underground jungle. Spend every moment surrounded by blocks on all sides, as nearly every enemy can kill you in 2 hits from full HP with your current level of gear. If you get poisoned, it's possible for an enemy to kill you in one hit. Here's a picture showing a way to dig tunnels that lets you avoid dying from enemies falling on your head: https://i.imgur.com/an9l7ze.png
If you dig a 50+ block tunnel straight down and die, you can't come back and reuse the tunnel since you'd die from fall damage. The horizontal spaces exist to give you room to fight off enemies and place campfires to regenerate health. Feel free to make the horizontal tunnels wider to give yourself more room to fight enemies that drop down from above and fly in from below.
These tunnels are 20 blocks deep and 8 blocks wide. 20 blocks is a short enough distance that you won't take fall damage. Also, the reason why 2 vertical portions were dug going to the right, then 2 to the left, and would have repeated like that, is due to the way enemy AI works. If an enemy spawned above me, fell down, and tried to chase me, it would most likely get stuck on a wall trying to reach me. If I dug all of the tunnels going to the right or to the left, any enemy would be able to reach me. Another way to prevent this is to periodically wall up the paths behind you so that even if an enemy does make it into your tunnel, it'll be blocked by solid walls.
The only enemies that can pass through blocks to reach you are man eaters, but they have a limited range of movement, so they should never be able to reach you and damage you if you play slowly and carefully. http://terraria.gamepedia.com/Man_Eater
 * Craft an amazon. It's very good in the early game, and using it is a good way to conserve ranged weapon ammo. http://terraria.gamepedia.com/Amazon
 * Next, craft a thorn chakram. It's another fantastic weapon when you're just starting off, and it's also great for saving ranged weapon ammunition. http://terraria.gamepedia.com/Thorn_Chakram
 * Another one of your primary goals is to find bombs by breaking pots at the cavern layer or lower. The demolitionist NPC can only move in after you obtain bombs, and obtaining them randomly from pots in the cavern layer or lower is the fastest way to get them.
 * Finally, craft jungle armor. It'll take awhile to gather the materials, and it'll be tempting to run out into the open, surrounded by 5-10 enemies on all sides, trying to kill them with your fancy new chakram, but don't. That's a quick, stupid way to get yourself killed. Stay inside the tunnels you dig. Keep yourself surrounded by *solid blocks*, not enemies. Don't underestimate how much damage a single hit will deal to you, and don't underestimate how quickly new enemies spawn to take the place of dead ones. Remember: when accounting for poison damage, it's possible for an enemy to kill you in one hit from 100 HP. Only consider going out in the open once you've got at least 2 out of 3 pieces of the jungle armor set, or once you've got at least 140 HP & at least half a dozen healing potions, or some other combination of items/max HP that provides similar survivability.
http://terraria.gamepedia.com/Jungle_armor
 * Optionally, craft an ivy whip while you're here if you haven't gotten sick of the jungle yet. http://terraria.gamepedia.com/Hooks#Pre-Hardmode

Step 3:
 * Craft a gold or platinum watch. Lower tier watches aren't accurate enough.
 * After obtaining at least 30 bombs, preferably by buying most of them from the demolitionist, go to the crimson or corruption and destroy 2 crimson hearts or shadow orbs between 12:00 AM and 4:30 AM. You'll need to use the bombs to destroy the terrain to reach the hearts/orbs, and you're encouraged to craft at least 15 of them into sticky bombs to make this easier. (You could technically also use purification powder or a strong enough pickaxe, but they aren't available at this point of progression. By the way, this guide likes to pretend that the reaver shark doesn't exist since it allows you to skip about 1/3rd of the game's progression. That's too much IMO.) Destroying 3 hearts/orbs would spawn a boss, so you should only destroy 2 unless you're prepared to die. When you destroy them between 12:00 and 4:30 AM, each one has a 50/50 chance of instantly spawning a meteorite somewhere in the world each time. Destroying them at any other time delays the 50/50 meteorite spawn chance until the next day, and has the even worse drawback of only rolling *a single 50/50 chance* no matter how many hearts/orbs you break, rather than a 50/50 chance for *each* heart/orb you break.
http://terraria.gamepedia.com/Crimson_Heart
http://terraria.gamepedia.com/Meteorite_(biome)

 * After a meteorite lands, find it, harvest it, and craft meteor armor and a space gun. This set of gear is incredibly useful at the beginning of the game. You can mine meteorite with your gold/platinum pickaxe, but it's much faster and safer to use sticky bombs to blow up the meteorite ore and pick it up. Obtaining obsidian (also by using bombs) and crafting an obsidian skull will make this significantly safer, since the obsidian skull prevents you from taking fire damage whenever you touch meteorite ore, hellstone ore, and hellstone bricks.
http://terraria.gamepedia.com/Meteor_armor
http://terraria.gamepedia.com/Space_Gun
http://terraria.gamepedia.com/Obsidian_Skull

 * Build an arena. An arena is an area that's efficient for fighting bosses and surviving monster invasions. It uses wide, flat spaces and platforms to maximize mobility, as well as environmental buffs such as campfires and heart lanterns. Your first arena should be somewhat high above the ground so the eye of cthulhu won't be able to hide underground nearly half the time when it's dashing around during its 2nd phase.
 * It's always a good idea to build a small room to the side of your boss arenas in multiplayer with a bed, a light source, and background walls, then set your spawn point at the bed by right clicking it (if no message appears, you did it wrong). Encourage other players to set their spawn point there as well so you can all rejoin the fight much faster if you die.
 * Many bosses can only be fought at night, including the eye of cthulhu. You should always fight them as early in the night as you can, because when morning comes, these bosses either fly away rapidly and despawn, or gain a massive attack and defense boost that makes them almost invincible and allows them to kill you in one hit. The longer you wait at night to start the fight, the higher the risk that morning will come before you kill the boss.
http://terraria.gamepedia.com/Campfire
http://terraria.gamepedia.com/Heart_Lantern
 * Defeat the eye of cthulhu using meteor armor and a space gun, or weaker armor and a thorn chakram, or a gold/platinum bow with frostburn arrows. It'll drop T5 (tier 5) ores, which are useful for progression. Because this is an expert mode world, this boss will also drop a shield of cthulhu. It's very useful for fighting bosses, traveling across the overworld, and exploring underground, so it should be equipped at nearly all times.
Estimated difficulty: ★★☆☆☆
http://terraria.gamepedia.com/Shield_of_Cthulhu
 * If you don't want to rely on the space gun too much, craft a tendon or demon bow using the T5 ores that were dropped by the eye of cthulhu. http://terraria.gamepedia.com/Tendon_Bow
http://terraria.gamepedia.com/Demon_Bow

Step 4:
 * Use a gravitation potion you've surely obtained from breaking pots by now to find floating islands, and find a starfury. Finding a lucky horseshoe would be a nice bonus, but it isn't strictly needed. Rather than using starfury as a weapon, it'll be used from this point onward to find life crystals and underground cabins much faster than usual by lighting up caves. Every time you use starfury, three stars fall from the top of the screen and light everything up on their way down. Due to harpies having the ability to kill a 100 HP player in 2 hits and relentlessly hunting down players without the shield of cthulhu (because they can't dash and don't have wings in pre-hardmode, and thus aren't fast enough to get away from the harpies), you should complete most of the steps above before you try this. At the very least, get a full set of meteorite armor and a space gun so you can kill the harpies before they kill you. Harpies won't spawn in the middle 1/3rd of the map before hardmode, so search there first.
http://terraria.gamepedia.com/Gravitation_Potion
http://terraria.gamepedia.com/Floating_Islands
http://terraria.gamepedia.com/Starfury
http://terraria.gamepedia.com/Lucky_Horseshoe
 * Go to the underground jungle and bring at least half of the spelunker potions you've found from pots and chests. Using those + the meteorite/space gun set to defend yourself and starfury to light up the caves, find enough life crystals to get 400 HP. Share extras with other players, as always, or craft them into heart lanterns for boss/event arenas. You should also bring the materials to craft more spelunker potions (sand/glass/bottles, gold/platinum ore, blinkroot, and moonglow) on the fly. http://terraria.gamepedia.com/Spelunker_Potion
 * Find hermes boots or flurry boots (or sailfish boots if you don't mind the boredom of fishing), and a magic mirror if you haven't already.
http://terraria.gamepedia.com/Hermes_Boots
http://terraria.gamepedia.com/Flurry_Boots
 * Acquire any type of hook if you haven't already. Slime hooks, amethyst hooks, topaz hooks, and ivy whips are the 4 easiest types to obtain. http://terraria.gamepedia.com/Hooks
 * Optional: find a staff of regrowth in the underground jungle. It's extremely useful for harvesting herbs from your herb farm, but finding one is pure RNG.
http://terraria.gamepedia.com/Staff_of_Regrowth
http://terraria.gamepedia.com/Planter_Box

 * Eventually defeat the king slime when it spawns at the end of a slime rain event. (You don't have to bother making slime crowns to spawn it manually if you don't want to.) After placing 31-35 ropes in a vertical line, it's easy to defeat him by climbing to the top, placing about 10 wooden blocks above your head horizontally to prevent falling slimes from knocking you off the rope, and shooting down at the king slime until it dies. Neither the boss nor the other slimes will be able to touch you during the fight if you stay at the top of the rope. Summoning the king slime with a slime crown and beating it repeatedly is one good way to farm money. The same can be said of the eye of cthulhu, especially if you craft bars from the T5 ores it drops, then sell them.
Estimated difficulty: ★☆☆☆☆
http://terraria.gamepedia.com/King_Slime
http://terraria.gamepedia.com/Slime_Rain
http://terraria.gamepedia.com/Slime_Crown
https://www.youtube.com/watch?v=yHBCPUYp-j4&t=2m22s
 * Obtain more wood and improve the size of the NPC house if it still isn't large enough to house 20 NPCs (21 including Santa during the holiday season). There are 23 NPCs that can move in (including Santa for about 1 month out of the year), but 2 of their houses will need to be made in different biomes later on in the game, so don't worry about those for now.
 * Buy planter boxes from the dryad and build an herb farm. Gather whichever seeds you're missing. Make a bunch of spelunker potions, among other things. For convenience, the rows of deathweed, fireblossom, and shiverthorn should be the easiest to access/closest to your spawn point, blinkroot and daybloom should be farthest away, and waterleaf and moonglow should be somewhere in-between. Fireblossom will be obtained later.
http://terraria.gamepedia.com/Spelunker_Potion
http://terraria.gamepedia.com/Planter_Box
http://terraria.gamepedia.com/Deathweed
http://terraria.gamepedia.com/Fireblossom
http://terraria.gamepedia.com/Shiverthorn
http://terraria.gamepedia.com/Blinkroot
http://terraria.gamepedia.com/Daybloom
http://terraria.gamepedia.com/Waterleaf
http://terraria.gamepedia.com/Moonglow

Step 5:
 * Build an arena for the brain of cthulhu. If the world has corruption instead of crimson, we'll fight the eater of worlds instead and an arena will be unnecessary. It bears repeating that campfires and heart lanterns are extremely beneficial. The amount of HP they recover during 60+ second long fights can make the difference between winning and losing.
This is some of the best gear to use for this fight, in order from best to worst:
#1. Space gun, meteor armor
#2. Bone javelins, fossil armor
#3. Gold or platinum bow, frostburn or unholy arrows (the latter are better for this fight, but they're only available on crimson worlds after beating the brain of cthulhu), any armor
http://terraria.gamepedia.com/Brain_of_Cthulhu
http://terraria.gamepedia.com/Eater_of_Worlds
http://terraria.gamepedia.com/Fossil_armor
http://terraria.gamepedia.com/Bone_Javelin
http://terraria.gamepedia.com/Unholy_Arrow

 * Destroy a 3rd crimson heart and defeat the brain of cthulhu. Hopefully one of the 3 crimson hearts will drop a rotted fork or a crimson rod. The rotted fork is good for the brain's 2nd phase, and the crimson rod is good for both the 1st and 2nd phase.
Estimated difficulty: ★★☆☆☆
http://terraria.gamepedia.com/The_Rotted_Fork
http://terraria.gamepedia.com/Crimson_Rod
 * Or destroy a 3rd shadow orb and defeat the eater of worlds. The strategy in this video is no longer 100% effective, but it's still extremely good and trivializes the fight.
Estimated difficulty: ★★☆☆☆
https://www.youtube.com/watch?v=j4ILiiKu3vQ&t=10s
 * Craft T5 armor, then defeat the brain of cthulhu/eater of worlds a few more times for additional items—the most important ones being nightmare or deathbringer pickaxes—and share them with other players.
http://terraria.gamepedia.com/Tissue_Sample
http://terraria.gamepedia.com/Deathbringer_Pickaxe
 * Hopefully defeat a goblin army by this point, find and rescue the goblin tinkerer, and purchase rocket boots & a tinkerer's workshop.
http://terraria.gamepedia.com/Goblin_Tinkerer
http://terraria.gamepedia.com/Rocket_Boots
http://terraria.gamepedia.com/Tinkerer%27s_Workshop

Step 6:
 * Go to the underground jungle and make a life fruit farm. You'll be *really* glad that you made it in late pre-hardmode, rather than pre- or post-Plantera when the enemies are much more dangerous. (Hell, hardmode jungle enemies are still pretty deadly even when you have post-Golem gear.) Make 1 to 4 horizontal tunnels that are anywhere from 100 to 400 blocks long and your life fruit farm will eventually bear fruit later on in hardmode. I typically make 2 ~200 block long tunnels that are only separated by 1 long, thin row of mud blocks. Make sure your tunnels are deep enough underground that they count as being in the underground jungle, not the surface jungle, or else life fruit won't spawn in them. http://terraria.gamepedia.com/Life_Fruit
 * Dig a hellevator that leads all the way to the underworld.
 * Obtain at least 1,000 stone, dirt, or snow blocks along the way. Later, these will be used to make a bridge.
 * Build a somewhat safe building, including a bed to respawn in, at the bottom of the hellevator. Don't forget to make the landing area safer by using pink slime blocks, assuming you have some pink gel by this point. Landing in water is unreliable and sometimes results in dying from fall damage (which is a glitch), or going AFK while falling down the hellevator, forgetting how deep the water below is, and coming back to see that you've drowned. http://terraria.gamepedia.com/Pink_Slime_Block
 * Make the area below the hellevator rest building safer by adding platforms and campfires, as well as peace candles if any are available.
 * Obtain some fireblossom and expand the herb farm. It'll be especially useful for crafting obsidian skin potions, which make hell/the underworld much, much safer. http://terraria.gamepedia.com/Fireblossom
http://terraria.gamepedia.com/Obsidian_Skin_Potion
 * Obtain T8 armor, tools, and weapons, including an imp staff, and craft an obsidian skull if you haven't already. The primary goal is to collect enough molten armor and imp staffs to share with everyone. Molten armor makes a huge difference in the next three boss fights.
http://terraria.gamepedia.com/Hellstone
http://terraria.gamepedia.com/Molten_armor
http://terraria.gamepedia.com/Imp_Staff
 * You've most likely found an underground mushroom biome by this point. If not, go find one. Destroy some glowing mushrooms until you've got a few mushroom grass seeds. After that, return to the surface, place about 100-200 mud blocks relatively close to spawn (but not too close - up in the sky is a good place for this), and plant your mushroom grass seeds in the mud. Eventually, the mushroom grass will spread to all the mud blocks that are touching one another & air blocks. (Mud blocks that are surrounded by other solid blocks on all sides won't be converted into mushroom grass blocks.) When there are enough mushroom grass blocks in the area, this area will be considered a surface mushroom biome. Somewhat dangerous mushroom enemies will be able to spawn nearby if you ignored the advice in this paragraph and built this area on the ground instead of in the sky, so in that case, be careful.
Either way, build an NPC house nearby and make sure that no NPC lives in it for now. In mid to late hardmode, a special NPC will be able to move into this house since it's in a surface mushroom biome. To make an NPC move into a different house, stand near the room/house you want it to move into. Open your inventory and click the tiny icon of a house above your social/vanity armor slots. Lastly, click the icon of the NPC you want to move, then click the empty room/house you want it to move into. After you move away so that the NPC and the room are both about 10-20 blocks or more offscreen, the NPC will have teleported into its new home. http://terraria.gamepedia.com/Truffle
 * Now would also be a good time to go to an underground mushroom biome and expand it by adding many rows of mud blocks and connecting them to mushroom grass blocks, so that the mud blocks will eventually be converted into mushroom grass blocks. If you do this now, you'll save yourself a lot of trouble dealing with much deadlier enemies in hardmode. This underground mushroom biome will be your truffle worm farm later in the game.

Step 7:
 * Build another boss arena above the jungle for the queen bee. Fighting her underground makes the fight at least twice as hard and take nearly twice as long due to the player having limited mobility and the queen bee hovering inside solid blocks for much of the fight, making her invincible during those times. Part of the arena should have a roughly 3 block wide pool of honey the player can step into to boost their life regeneration for 30 seconds afterward.
This is some of the best gear to use for this fight, in order from best to worst:
#1. Minishark, any type of bullets, molten armor. If you don't have enough gold to buy a minishark, you can either kill the eye of cthulhu 1-4 times and sell the items it drops or use one of the money-making methods listed here: http://terraria.gamepedia.com/Guide:Making_money#Collecting
If you use molten armor you'll hardly take any damage. You could reasonably do this fight with only 100 HP using this gear, especially if you use buff potions before the fight starts and equip several warding accessories.
#2. Molten fury, frostburn arrows, molten armor
#3. Bone javelins, molten armor
#4. Space gun, meteor armor. This not only takes several minutes longer to kill the queen bee than the item sets above, you'll also take significantly more damage during the fight. Players who have never fought the queen bee before and use this set of gear might die. Experienced players using buff potions probably won't, however.
http://terraria.gamepedia.com/Honey
http://terraria.gamepedia.com/Molten_Fury

 * Use an abeemination to summon the queen bee aboveground, then defeat her at the jungle arena multiple times for the great items she drops.
Estimated difficulty: ★★☆☆☆
http://terraria.gamepedia.com/Queen_Bee
 * Build a small NPC house for the witch doctor after defeating the queen bee. In hardmode, he'll sell better items if his house is in the jungle. http://terraria.gamepedia.com/Witch_Doctor

 * Build an arena for skeletron and defeat him. Whoever builds the arena should add an easily accessible honey pool one block below its lowest line of platforms, and make it 2 blocks wide. That's so small that you can easily jump over it when you don't want to walk through it, and you'll only be standing in it for a tiny fraction of a second, so it won't slow you down enough to make you take a hit that you otherwise would've dodged. The extra 60 HP recovery per minute (240 bonus HP if the fight lasts 4 minutes, 300 if it lasts 5 minutes, etc.) helps immensely.
The arena builder should also make the arena at least 50% wider than their usual arenas, and it should only have 2 rows of platforms - no more, no less. The more rows they add, the more chaotic the fight becomes as everyone flies around in different directions, making skeletron move in unpredictable directions toward random players as he changes targets.
It bears repeating that campfires and heart lanterns are extremely beneficial. The amount of HP they recover during 60+ second long fights can make the difference between winning and losing.
This is some of the best gear to use for this fight, in order from best to worst:
#1. The bee's knees, wooden arrows, molten armor, and beenades for the 2nd phase. When using bee weapons, you should always use a hive pack. Using a shark tooth necklace is also a good idea.
#2. Molten fury, frostburn arrows, molten armor, and beenades for the 2nd phase
#3. Minishark, silver bullets or meteor shot, molten armor, and beenades for the 2nd phase
#4. Demon scythe and molten armor, or a space gun and meteor armor, and beenades for the 2nd phase. I don't recommend using either of these setups, but I included them anyway for people doing pure mage playthroughs. Also, you'll need to reforge your space gun to have at *least* a +10% damage bonus *and* use a shark tooth necklace. If you only use one or the other or, god forbid, neither one, the fight will take a painfully long time, easily adding *multiple minutes* to the fight. This is especially awful because of how low your defense is when using space armor compared to molten armor; the longer the fight drags on, the more likely you are to lose. Seriously, don't use these sets unless you're doing a magic-only run. For comparison: the fight can last 1 minute and 50 seconds using the bee's knees and wooden arrows, or it can last 3 minutes and 35 seconds using a demon scythe, and that's *with* the 2 buffs (+10% damage from the "deadly" prefix + a shark tooth necklace) I mentioned earlier?

No other gear is recommended. All of the other sets listed prior to this step, including the bee gun, were tested against skeletron multiple times and were found to be noticeably less effective than the sets recommended above. Note: all testing is done without any buff potions so your mileage may vary, especially depending on which accessories you use, which modifiers they have, which modifiers your weapons have, how accustomed you are to fighting these bosses, etc.
You're strongly encouraged to destroy both of skeletron's hands at roughly the same time by damaging them at about the same rate throughout the first phase of the fight. As soon as one of his hands is destroyed and skeletron's skull isn't spinning, he'll start firing homing skulls at you. This gives you the benefit of only having to worry about skeletron's skull and its homing mini-skulls, rather than all of those plus one of his hands at the same time.
After destroying skeletron's hands, you should start using beenades to attack skeletron. Unlike most grenades, beenades don't deal damage to yourself when you're near the explosion. To hit most/all of your shots with beenades, *only throw them when skeletron stops firing skulls, slows down, and chases you in a straight line.* If you throw beenades while he's moving quickly and firing homing skulls, you'll likely miss most of the beenades you throw. They fall through platforms and are pretty useless when they blow up on the ground below you.
You should also fly around skeletron in circles during his second phase, when he starts shooting homing skulls at you. Not doing so makes it much more likely that you'll get hit by several of the homing skulls and die on the second phase.
Alternatively, you can position yourself below skeletron's skull and repeatedly dash left and right using a shield of cthulhu to confuse his targeting and dodge his homing skulls more easily.
How good or bad you are at using either of the 2 aforementioned mobility tactics during the 2nd phase of the fight will have the biggest impact on the fight's difficulty.

Estimated difficulty: ★★★★☆
http://terraria.gamepedia.com/Skeletron
http://terraria.gamepedia.com/Minishark
http://terraria.gamepedia.com/Shark_Tooth_Necklace
http://terraria.gamepedia.com/The_Bee%27s_Knees
http://terraria.gamepedia.com/Hive_Pack
 * Explore the dungeon. Make it safer by destroying spikes and water candles. Also place torches, platforms, and campfires. The torches and campfires allow players to see clearly, platforms increase mobility and reduce the risk of dying from fall damage, and campfires increase HP regeneration when players are nearby.
http://terraria.gamepedia.com/Dungeon
http://terraria.gamepedia.com/Spike
http://terraria.gamepedia.com/Water_Candle
 * Get 1+ bewitching table(s) from the dungeon. It provides a buff that lets you summon one additional minion, such as imps from the imp staff or wasps from the hornet staff. Alchemy tables are also useful and can only be found in the dungeon.
http://terraria.gamepedia.com/Bewitching_Table
http://terraria.gamepedia.com/Alchemy_Table

Step 8:
 * Build a bridge out of at least 2,000 platforms to fight the wall of flesh. Incorporate the floors and ceilings of the demon apartment buildings to make the bridge a couple hundred blocks longer. Using platforms instead of solid blocks prevents a frequently occurring glitch where the WoF can grab you and usually releases you underneath the bridge, often over lava or inside solid piles of ash. This can lead to a nearly instant death. If you have reason to believe that some players won't contribute to the fight, either due to being AFK or insisting on using absolutely terrible (1-50 DPS) weapons against the wall of flesh, extend the bridge another 500 blocks for every player who isn't really contributing.
http://terraria.gamepedia.com/Wall_of_Flesh
 * Defeat the wall of flesh. Piercing weapons are strongly recommended due to the boss spawning many small enemies in front of itself regularly during the fight. Its eyes have less defense than its mouth, so players should aim for one of the eyes whenever possible. Here are some of the best weapons to use for this fight, in order from best to worst:
#1. The bee's knees + wooden arrows or the bee gun, necro or molten armor if using the bee's knees, jungle or molten armor if using the bee gun. Use necro/jungle if you're confident in your ability to dodge most of the lasers. Otherwise, use molten. Unlike when you fought skeletron, the bee's knees and the bee gun perform equally well in this fight due to the large difference between these two bosses' defense and other factors. When using bee weapons, you should always use a hive pack. Using a shark tooth necklace is also a good idea.
#2. Minishark or phoenix blaster, meteor shot, necro or molten armor. Without meteor shot, this gear is pretty bad for this boss fight. Pre-hardmode, meteor shot are the only type of bullets that can pierce enemies, and due to the high number of enemies that constantly spawn in front of the boss, you'll need to be able to pierce them to contribute to the fight.
#3. Bone glove, bones, fossil armor
#4. Demon scythe, meteor or jungle armor
You can also use dynamite to cheese this fight, but that's overkill, and if you mess up and lose the fight, you'll have to spend time repairing the holes you blasted in your bridge.

Estimated difficulty: ★★★☆☆
http://terraria.gamepedia.com/Phoenix_Blaster
http://terraria.gamepedia.com/Meteor_Shot
http://terraria.gamepedia.com/Bone_Glove
http://terraria.gamepedia.com/Bone
http://terraria.gamepedia.com/Sticky_Dynamite

Step 9 (Hardmode begins):
 * Try not to die too much. After defeating the wall of flesh, the world enters hardmode. In hardmode, normal enemies become very powerful. New types of enemies will spawn, and they are also very strong. Many deaths will probably occur around this time.
 * Get 2+ golden showers and share the extras with other players if the world is crimson. It's an extremely useful magic weapon. After accounting for its defense reducing ability, it deals roughly 339 DPS. That's without modifiers, accessories that boost damage, and so on. If the world has corruption, I'll focus on getting 2+ crystal storms (214 DPS) and sharing them instead. It would also be great to get a meteor staff (333 DPS) since they're good against the 3 upcoming bosses, especially the destroyer, but they're terrible when used underground, and there might not be enough meteorite leftover from pre-hardmode to make one.
http://terraria.gamepedia.com/Golden_Shower
http://terraria.gamepedia.com/Crystal_Storm
http://terraria.gamepedia.com/Meteor_Staff
 * Quickly obtain spider armor, a spider staff, and a queen spider staff.
http://terraria.gamepedia.com/Spider_armor
http://terraria.gamepedia.com/Spider_Staff
http://terraria.gamepedia.com/Queen_Spider_Staff

 * Get T9 and T10 pickaxes, craft a T10 anvil, craft a T11 forge, then get T11 gear.
http://terraria.gamepedia.com/Ores
http://terraria.gamepedia.com/Mythril_Anvil
http://terraria.gamepedia.com/Orichalcum_Anvil
http://terraria.gamepedia.com/Hardmode_Forges
 * Buy leaf wings from the witch doctor. They reduce all fall damage to 0, which makes everything safer. I'll upgrade to frozen wings later on.
http://terraria.gamepedia.com/Leaf_Wings
http://terraria.gamepedia.com/Frozen_Wings
http://terraria.gamepedia.com/%E3%82%A2%E3%82%AF%E3%82%BB%E3%82%B5%E3%83%AA%E3%83%BC#.E7.BF.BC
 * Collect souls of night and souls of light, craft them into keys, use them to spawn mimics, and kill them to obtain good items.
http://terraria.gamepedia.com/Soul_of_Night
http://terraria.gamepedia.com/Soul_of_Light
http://terraria.gamepedia.com/Key_of_Night
http://terraria.gamepedia.com/Key_of_Light

 * Find a philosopher's stone and craft a charm of myths. Bands of regeneration have a high random spawn chance, so I will probably already have 1-3 of them before hardmode. If I don't have the supplies necessary to make a meteor staff, then I should get a daedalus stormbow at this time.
http://terraria.gamepedia.com/Philosopher's_Stone
http://terraria.gamepedia.com/Charm_of_Myths
http://terraria.gamepedia.com/Daedalus_Stormbow
 * Start gathering supplies for greater healing potions. http://terraria.gamepedia.com/Greater_Healing_Potion
 * Craft a fairy bell or a similar item to summon a light pet if I don't already have one by this point. Light pets are pets which emit light. They are very useful when exploring underground. http://terraria.gamepedia.com/Fairy_Bell
 * Get a nimbus rod. I have no control over their spawn conditions and low drop rate, so this may occur at a later time such as Step 10, or as early as Step 8. Since it's very useful for the next boss fight, I prefer to put it off until after I obtain a nimbus rod. http://terraria.gamepedia.com/Nimbus_Rod
 * Defeat the destroyer. For more information on gear you should consider using and arena tips, check out this picture: https://i.imgur.com/ykfZOl2.png
It bears repeating that campfires and heart lanterns are extremely beneficial. The amount of HP they recover during 60+ second long fights can make the difference between winning and losing.
Estimated difficulty: ★★☆☆☆
http://terraria.gamepedia.com/The_Destroyer

Step 10:
 * Craft a megashark and light discs. You only have to kill the destroyer once to craft both of these on expert mode, but multiple light discs can be made and stacked in the same inventory slot to increase their effectiveness.
http://terraria.gamepedia.com/Megashark
http://terraria.gamepedia.com/Light_Disc
 * Farm the destroyer a few times for hallowed bars, which can be used to make hallowed armor, among other things.
http://terraria.gamepedia.com/Hallowed_Bar
http://terraria.gamepedia.com/Hallowed_armor
 * Return to the jungle and obtain 20+ life fruits to reach 500 HP. Share extras with other players. Life fruits only spawn after 1 mechanical boss has been defeated, but they're very rare up until you defeat all 3 mech bosses.
 * Try to get the witch doctor to live in a house in the jungle. http://terraria.gamepedia.com/Witch_Doctor
 * Find a frozen turtle shell. http://terraria.gamepedia.com/Frozen_Turtle_Shell
 * Defeat the twins and skeletron prime in any order.
Estimated difficulty: ★★★☆☆
http://terraria.gamepedia.com/The_Twins
http://terraria.gamepedia.com/Skeletron_Prime
 * Craft a drax. http://terraria.gamepedia.com/Drax

Step 11:
 * Return to the jungle and obtain chlorophyte bars. http://terraria.gamepedia.com/Chlorophyte_Bar
 * Buy an autohammer and craft shroomite gear.
http://terraria.gamepedia.com/Autohammer
http://terraria.gamepedia.com/Shroomite_armor
 * Create an arena for plantera and consider defeating it multiple times since it drops decent items. If the arena has plenty of wide, open space, the fight will be significantly easier.
Estimated difficulty: ★★★☆☆
http://terraria.gamepedia.com/Plantera
 * Preferred item drops from plantera:
http://terraria.gamepedia.com/Pygmy_Staff (50% chance of obtaining this when plantera dies)
http://terraria.gamepedia.com/Leaf_Blower (1/7 chance)
http://terraria.gamepedia.com/Nettle_Burst (1/7 chance)

 * The dungeon becomes much more difficult after plantera dies for the first time. New enemies spawn, and they drop new items. Return to the dungeon and obtain these items. Some examples include:
http://terraria.gamepedia.com/Spectre_armor
http://terraria.gamepedia.com/Inferno_Fork
http://terraria.gamepedia.com/Paladin's_Hammer
http://terraria.gamepedia.com/Kraken
 * Enter the jungle temple and defeat the golem multiple times. He drops good items, but he's weaker than plantera for some reason.
Estimated difficulty: ★★☆☆☆
http://terraria.gamepedia.com/Golem
 * Attempt to survive every wave of the pumpkin moon and frost moon events. The pumpkin moon is easier, so I'll start that one first.
http://terraria.gamepedia.com/Pumpkin_Moon
http://terraria.gamepedia.com/Frost_Moon

Step 12:
 * Create an arena for duke fishron and defeat it multiple times until it drops a tempest staff, and either a tsunami or a flairon.
Estimated difficulty: ★★★★☆
http://terraria.gamepedia.com/Duke_Fishron
http://terraria.gamepedia.com/Tempest_Staff
http://terraria.gamepedia.com/Tsunami
http://terraria.gamepedia.com/Flairon
 * Create a very long minecart track to fight the moon lord if one hasn't been made already.

Step 13 (All of this must be done together, in this order. Defeating the cultist immediately begins the lunar events, and destroying the final celestial tower spawns the moon lord after 60 seconds):
 * Improve the old arena for skeletron (if necessary) and defeat the lunatic cultist.
Estimated difficulty: ★★★☆☆
http://terraria.gamepedia.com/Lunatic_Cultist
 * The lunar events will immediately begin. Destroy the 4 celestial towers.
Estimated difficulty: ★★★☆☆
http://terraria.gamepedia.com/Lunar_Events
 * Craft a solar eruption early on to make the remaining pillars easier to defeat. http://terraria.gamepedia.com/Solar_Eruption
 * Craft a phantasm before the moon lord spawns and use it to defeat him. http://terraria.gamepedia.com/Phantasm
 * The moon lord will spawn 60 seconds after the fourth celestial tower is destroyed. Defeat him.
Estimated difficulty: ★★★★★
http://terraria.gamepedia.com/Moon_Lord
 * If we lose, at least we can craft better gear using the items we obtain from the 4 celestial towers to improve our chances of winning the next time.
http://terraria.gamepedia.com/Stardust_Dragon_Staff
http://terraria.gamepedia.com/Phantasm
http://terraria.gamepedia.com/Solar_Eruption
http://terraria.gamepedia.com/Nebula_Blaze


Sometime between steps 1 and 7, I hope to obtain the following items. I have little control over when I will find them. I might not find some of them, such as a metal detector, due to their random spawn conditions.
 * An ammo box, any type of counterweight, a lifeform analyzer, spelunker glowsticks, and a magic lantern. These are sold by traveling merchants and skeleton merchants, which appear at random. Skeleton merchants are somewhat rare.
http://terraria.gamepedia.com/Traveling_Merchant
http://terraria.gamepedia.com/Skeleton_Merchant
 * A metal detector. You can only obtain a metal detector by defeating a Nymph. Nymphs are rare enemies. I might perform steps 1-7 and not encounter a Nymph. It's completely random. Anyway, metal detectors are extremely useful. They greatly speed up progression through the game.
http://terraria.gamepedia.com/Nymph
http://terraria.gamepedia.com/Metal_Detector
 * Mana crystals. These are crafted by combining 3 fallen stars. Fallen stars only spawn at night, they only spawn on the surface of the world, and their spawn locations are random.
http://terraria.gamepedia.com/Fallen_Star
http://terraria.gamepedia.com/Mana_Crystal
 * A money trough. You can only obtain a money trough by defeating blood zombies and dripplers, which only spawn during blood moons. The occurrence of blood moons is random. Additionally, those enemies only have a 1% chance of dropping a money trough when they die. http://terraria.gamepedia.com/Money_Trough