The point of this guide is to give you a headstart into the game, these are all things that you'd typically learn during your first 100 hours or so of gameplay (most people never actually get around to learning lots of these things beyond the most basic ones - we call those people "turboshitters").
Hopefully you'll start sucking a little bit less soon enough.
This guide assumes that you have read the other one (http://pastebin.com/CWn3VQJD) and that you are familiar with basic terminology.
--- I have some certs, what do I buy?
Until BR 15, you get 100 certs every time you rank up. That's right, completely for free. Now, once this initial phase of cert abundance is passed, it will take you about 3-4 hours to get 100 certs, so it's very important that you don't waste your welfare certs. If you're not sure, ask in the thread.
You are automatically given a few attachments for your starter weapons, make sure to equip them. (These are good, there's no need to "upgrade" to something else in almost every case. As a general rule, when unlocking a new weapon of the same type, you'll want to use the same attachments as those that were given to you on your starters). Some weapons have access to high velocity ammo (HVA) or soft point ammo (SPA): you'll generally want SPA, unless you know you'll be fighting at long range and that short range engagements are unlikely.
Medkits are very useful, as they allow you to refill your health instantly (note, not your shield). Restoration kits slowly refill your health over time, and are generally seen as inferior (although they do have half the resource cost medkits do).
You may be tempted to get restokits instead of medkits because of their lower cert cost, but all ranks besides the first one have the same cost.
Get medkits, those things will give a huge boost to your survivability. Two or three is enough, but if you happen to have enough certs laying around, get the fourth medkit. Medkits (and restokits) are unlocked for all classes.
Once you unlock medkits or restokits, any class can use them.
However, C4 only gets unlocked on one specific class. Most people unlock it on LA first, because the jetpacks allow them to dunk on vehicles with ease, or on medic, because a medkit can be redundant if you're using the AoE heal (read: self heal).
C4 is an extremely valuable tool, since one brick OHKs infantry, flashes, harassers, turrets, MAXes (without flak armor), ESFs and valks (although these two do take some real skill), and two bricks will OHK a lightning, an MBT, a MAX with flak armor, and do a sizeable chunk of damage to sundies.
C4 is rather expensive, costing 200 certs for the first brick and 500 certs for the second one, but it is worth it.
NEARLY ALL STARTER WEAPONS ARE GOOD. This can not be stressed enough. That 1000 cert gun isn't an upgrade over your current gun, but merely a sidegrade. All guns are sidegrades. The general rule is that, the more a gun differs from that class' starter, the higher the cert cost.
The only real exception to this is the Gauss SAW. This is the NC HA's starting LMG, it has strong vertical recoil, but more importantly it has huge CoF growth. Experienced players can make the most of it because of its high damage and starting accuracy, but it is strongly recommended that you get the 325-cert GD-22S if you want to play HA.
Get the empire specific anti-air lock on rocket launcher, especially if you find yourself playing HA a lot. These have slightly lower damage than your default RL when dumbfired, but they lock on to enemy aircraft (duh) within a certain range.
A2G air can be really annoying to ground troops, and even only getting the lock warning will make them fuck off for a bit. In case you're wondering, it takes between two and three G2A rockets to kill an ESF.
To deal with ground vehicles or MAX units, you can choose to use the Decimator. It has slightly higher damage than the starter RL, but has longer reload, lower velocity and carries one less rocket. The Decimator also OHKs ESFs, so if you think you're very good at leading targets you'll want to use it.
Buying burst-fire weapons often seems like a good idea to new players that got bored of their current gun; it is not. Those weapons, while potentially very effective in the hands of an experienced player (especially now that they got huge buffs), are tougher to use, and require a complete shift in the way you use a gun to be used properly. If you buy a burst-fire weapon at this stage, you're likely wasting your certs and your time.
--- What class should I play?
As a newb, you'll want to stick to support classes at first. Compared to heavy or light assaults of similar skill, a medic prints certs like mad (engineer a bit less, but ARs, which are medic-exclusive, statistically are the best guns in the game, so that's a big help). One of the first things you should cert into is your Jesus gun: the first few ranks are cheap, then it gets more expensive, the last rank costing 500 certs. Don't be afraid, this thing will pay back for itself in no time. (hold left click to heal, hold right click to revive)
Remember that you're a combat medic, not a healbitch who occasionally kills somebody. When in a fight, first you make sure the enemy is dead, or at least that you're behind cover, and then revive dead friendlies.
Speaking of reviving, learn when reviving somebody is a bad idea. This will come with time through playing, but the general idea is that if they'll get killed again really soon (for example, if they have died in the middle of a busy doorway, they'll likely die again as soon as you res them) you shouldn't revive them.
Even if you decide not to play medic, stick to one class at first (ask for advice in the thread as to how to best play that class). Certs can be ridiculously hard to earn when you're first starting out, and spreading certs out between classes, instead of focusing on one specific class, is definitely not a good idea.
Don't play infiltrator.
There are several reasons for this. Either you'll play SMG inf, meaning that you'll abuse faulty netcode and kill players while you still appear cloaked to them, or you'll use a BASR and snipe from far away, meaning that you'll just be a major annoyance to enemies while occasionally getting a kill.
This will not help you improve anyhow, since sniping doesn't pose any kind of threat to you (besides potentially getting countersniped), and close-quarters infiltrator doesn't translate well to other classes, since the only way to make it work is abusing the cloak. Keep in mind that, at this stage, you're trying to get better, not to get cheap kills.
However, if you're playing with a group, playing infiltrator can have some advantages, such as using EMP grenades (which destroy enemy deployables and weaken enemies (by removing their shields), as well as disabling their HUD) or spamming motion sensors, which give your team valuable info.
---How do I get better?
Just like pretty much any FPS, your mouse sensitivity needs to be low, to ensure that your crosshair doesn't go all over the place when tracking an enemy. There isn't really a fixed value, but it should take you at least 20 cm to do a 360° hipfire rotation (possibly more, depending on your desk and mousepad size).
(No, really, 20 cm isn't good, it is bare minimum.)
Make sure that you are using raw mouse input, and that mouse accleration is disabled.
Be aware of your surroundings; occasionally look at the map for spot markers, use a pair of non-shit headphones (don't need to be all that good), make a note of where people are and where they could be going ("that guy is in that room and he's probably coming to the cap point, I could ambush him outside this door", etc). What weapons is an enemy using? Did he probably take damage from somebody else already?
You aren't the main character in an action movie and this is not a singleplayer game, you die just as easily as anybody else. Don't blindly rush rooms, cap points and so on. Learn to outsmart your opponents, and eventually to outshoot them.
One mistake newbs always do is assume that they're dying because the game is P2W: it is not, it's P2notgrind. And even then, the equipment gap is not that bad: statistically, given the same infantry class (this isn't true for vehicles), a fully certed character is only ~10-30% better than a new one (it used to be much worse, but new players now get loads of free shit. thank u werl). In fact, an experienced player on a stock character will ALWAYS win against a new player on a character with the "best" builds maxed out.
Whenever you die, take some time to think about why you died. Were there too many enemies? Did you stay out of cover for too much time? Did you try to 1v1 a MAX without explosives?
Then, make a conscious effort not to repeat that same error again.
You generally should run on your own, since being around zergs (read: bad players) won't make you improve anyhow. When possible, look for 24v24 - 48v48 fights, as those tend to be decent sized without being huge clusterfucks. If you notice that the fight you're at has died down or simply doesn't suit your taste, press U to redeploy and spawn somewhere else.
If you play at low-pop times, consider joining a squad to find a fight.
If you find one that suits your taste join an outfit (which is just a fancy word for clan). Playing with the same group of people regularly tends to build up a relationship of mutual trust, which is an extremely good thing, seeing as in this game you can't even rely on an average friendly to not "accidentally" shoot you in the head.
Do not walk to other bases.
Just walking around or cruising in a vehicle can be really comfy, but in the long run it'll be a huge waste of time; it gets boring quick. To get to a fight, learn to redeploy hop, or take an ESF.
Don't try to get into flying. It is long and painful process and frustrating as hell. Even if you manage to become proficient (which will take at the very least 100 hours of flying), don't expect to catch up with people who have been flying for years now, unless you practice a few hours a day literally every day. Knowing the basics can be very useful nonetheless, there are some good tutorials on youtube.
Also, flying physics are nothing like what you would expect them to be. All of the air game essentially revolves around a bug which has never been fixed (and never will), which allows you to fly backwards while shooting at your enemy. This has defined the air game, much like Quake's bunnyhopping and strafejumping has defined the gameplay in that game.
Tanks require comparatively huge cert investments, so don't bother until you get the infantry game sorted out. However, if you see a tanker that seems to know what they're doing, always try to hop in their gunner seat, possibly as an engineer.
If you get killed by a seemingly OP thing, such as a vehicle, a MAX, a sniper rifle or a shotgun, resist the urge to use one yourself. If you pull a vehicle out of rage, you'll likely get killed real quick.
Of course, you may pull vehicles or MAXes if you think it can be useful in this specific situation, but using something simply because you got killed by it is a terrible idea.