Welcome to the world of homelab. You have chosen a great starter server. And now that you have or are looking to buy your R710, what do you do with it? Here are some of the basics on the R710 and what you'll want to do to get up and running.
First we'll start off with the hardware...
CPU
The R710 has dual LGA 1366 sockets. They come stock with either Intel Xeon 5500's or Intel Xeon 5600's
One of the bigger things I see discussed here about the R710 is Gen I vs Gen II mainboards. The truth is for most intents and purposes, it doesn't matter. The only thing you'll be missing out on if you have a Gen I is any processor with 130TDP. If you check the 5600 series link above, you'll see there's only 5 processors that use 130W TDP. And these are not your regular run-of-the-mill processors. The cheapest X5690 on eBay currently runs about $180 each. If you absolutely need that kind of processing power, then sure, get a Gen II, but for most homelabbers, there's no need for any processor in the 130W TDP tier as they use more power and usually the processor will not be your first bottleneck on one of these servers. Most homelabbers here would recommend the L5640 as it has a TDP of 60W (Less than half of those processors needing a Gen II) and has 6 cores.

Memory
The R710 uses Up to 288GB (18 DIMM slots) of 1GB/2GB/4GB/8GB/16GB DDR3 800MHz, 1066MHz, or 1333MHz Registered (RDIMM) and Unbuffered (UDIMM).
There are lots of caveats to that statement though.
If you want the full 288GB, you'll have to use eighteen 16GB dual rank (more on this later) RDIMMs. The max UDIMM capacity is up to 24 GB (twelve 2 GB UDIMMs)
Now, the ranks on the memory matter. Each memory channel has 3 DIMM slots and has a maximum of 8 ranks each channel. So if you get 16GB quad rank DIMMs, you'll only be able to use 2 slots per channel bringing your maximum memory to 192GB. You'll be able to tell what the ranking of the memory is on the DIMM sticker. Here is a picture of what the sticker looks like. The rank will be indicated right after the memory capacity. So in this DIMMs case, it is a 2R or dual rank memory. You'll be able to to fill all 3 slots per channel with dual rank memory since the ranks will total 6 out of the maximum 8.
Another important thing about the memory on an R710 is that all channels must have the same RAM setup and capacity. You can mix and match RAM capacity as long as each channel has the same mix. For example, if channel one has an 8GB DIMM, a 4GB DIMM, and an empty slot, all other channels must have the same setup.
Yet another cavet of the memory is the speed. The R710 accepts memory speeds of 800MHz, 1066MHz, or 1333MHz. However, if you populate the 3rd slot on any of the memory channels, the speed will drop to 800MHz no matter the speed of the individual DIMMs.
Most homelabbers here would recommend to stick to 8GB 2Rx4 DDR3 1333MHz Registered DIMMS (PC3-10600R) This is the best bang for your buck on the used market. The 4GB DIMMs are cheaper, but will only give you a max of 72GB and if you want to go beyond that, you'll have to remove the 4GB DIMMS making them useless for your server. The 16GB DIMMS are about $50 each so if you fill up all 18 slots, it'll be about $900, ouch! The 8GB DIMMS should be cheap enough (~$14) to get a couple and get up and running, and give you enough space to grow if you max them out at 144GB.
One last thing about memory, you may find some PC3L memory. The L means it's low power. It runs at 1.35V if all other installed DIMMS are also PC3L. If any of the installed DIMMs are not PC3L, then they will all run at the usual 1.5V.
More info with diagrams can be found at the link below.
http://www.dell.com/downloads/global/products/pedge/en/server-pedge-installing-upgrading-memory-11g.pdf

RAID Controllers
The R710 has a variety of stock RAID controllers, each with their own caveats and uses.
SAS 6/iR, this is an HBA (Host Bus Adapter) it can run SAS & SATA drives in RAID 0, 1 or JBOD (more on this later).
PERC6/i this can run RAID 0, 1, 5, 6, 10, 50, 60 with SAS or SATA drives. It can not run in JBOD. It has a replaceable battery and has 256MB of cache.
These first two can only run SATA drives at SATA II speeds (3Gb/s) and can only use drives up to 2TB. So if you need lots of storage or want to run SSD drives, these would not be a good option. If storage and speed are not an issue, these controllers will work fine.
H200, this is also an HBA that is capable of RAID 0, 1, 10, or JBOD. It can use SAS & SATA drives.
H700, this can run RAID 0, 1, 5, 6, 10, 50, 60 with SAS or SATA drives. It can not run in JBOD. It has a replaceable battery and has either 512MB or 1GB of cache.
These two cards support SATA III (6Gb/s) and can use drive with ore than 2TB's. They are the more popular RAID controllers that homelabbers use on their R710.
Now, which to choose...
If you are planning or running a software RAID (ZFS, FreeNAS, etc..) then you'll want an HBA so that the OS can handle the disk. If you want a simple RAID, then the controllers with cache and battery backups will work better in that use case.
Another caveat, for the H200, if you want to run it in JBOD/IT mode, you will have to flash the firmware on the card. There are plenty of instructions out there on how to do this, but just make a note if that is your intention.

Hard Drives
Now that we have our RAID controller, we need something for it to control, HDD's.
The R710 comes in two form factors SFF (Small Form Factor, 8 - 2.5" drives) and LFF (Large Form Factor, 6 - 3.5" drives). Deciding between the two is up to you. 3.5" offer cheaper storage, 2.5" offers faster storage (SSD). If you're not sure which one to pick, you can go with the 3.5" as they have caddy adapters to use 2.5" drives on 3.5" caddies. Both form factors work the same so functionality will not differ.

iDRAC 6
iDRAC (integrated Dell Remote Access Controller) is exclusive to Dell servers (HP has iLO, IBM has IMM, etc...) it is a controller inside the server that enables remote monitoring of the server. There are two versions available for the R710.
iDRAC 6 Express, most servers come standard with this, but check to make sure the card wasn't removed. It can be used to monitor the servers hardware. It list all the hardware installed on the server and even lets your power the server on and off remotely. The express card should be located under the RAID controller on the mainboard.
iDRAC 6 Enterprise, this is a separate card that gets mounted to the mainboard near the back of the computer. It adds an additional network port specifically for connecting to the iDRAC. It also adds remote console, which means you can view everything that would output to the screen, including the BIOS, and you can use a keyboard and mouse to control what's on screen. This is very useful for remote troubleshooting, or just for not having to have a monitor, keyboard, or mouse connected to the server. The enterprise cards are pretty cheap on eBay (~$15) and are definitely recommended. One note, the enterprise card will not work on its own. It will also need to have the express card installed as well.

Power Supplies
The R710 has two different power supply options, 570W or 870W. The 570W PSU's are recommended for light loads. Xeon L or E processors, not too much RAM, not too many HDD's. If you're going to fill the chasis to the brim, go with the 870W version. Even if you're not going to be running much on it, the 870W gives you more room to grow, and does not use any more electricity that the 570W with the same load. All of the Xeon X processor need the 870W, same if you plan on filling all the DIMM slots. The 570W shouldn't be a deal breaker, unless you fall into the must have 870W use cases, but if you have a chance to pick up an 870W, it would be nice to have.
As far as dual PSU vs single PSU, in a home environment, it doesn't matter. Unless you can somehow connect the second power supply to a generator for when the power goes out, it's gonna be all the same. The only thing a dual PSU will protect you from is if the PSU fails which is quite rare. Again this shouldn't be a deal breaker, but if you can get dual PSU, why not, keep one as a spare.

Rails
This one is pretty simple. If you're planning on mounting the R710 in a rack, get them. If you're planning on having it o your desk, stuffing it in a closet, hanging it from the ceiling as a sex swing, no need for the rails.

Other
Some other questions frequently asked are...
Is it quite? It depends on your definition of quite and the load you're putting on the server. If you're trying to calculate the nth digit of pi, yea, it's gonna sound like a jet engine is taking off, but on idle it sounds about the same as an average gamming rig.
How much electricity does it use? Again, it depends. If you went with the low power CPU and only have 8GB of low voltage RAM, then not much. Luckily, one of our fellow homelabbers did some test on an R610 that you can look over to get a general idea of what to expect. https://www.reddit.com/r/homelab/comments/3d1w0b/a_comparison_of_power_draw_between_the_intel/
Is this a good deal [link]? This question gets asked alot. There's so many variables in a server that it's hard to pin down an exact price. Luckily someone has. Head on over to https://www.orangecomputers.com/node/?command=buildmodel&itemnum=PER710build&comp=Dell&model=Poweredge-R710-&ff=2U&config=PER710 Plug in the specs of the server you're looking at at it'll give you the price of what you can get one from this vendor. I've never bought from them, but they have pretty middle of the road pricing so it's a good guestimate to see if you're getting ripped off. Also, don;t forget to include shipping cost and these things are heavy and cost usually abour $50 to ship depending on origin and destination.
OK, that should be just about everything you need to know about the hardware and its quirks. Now to the next step.

Software
Now that you have an R710 with all the specs you want, ready to do what you need it to we can install... Wait! Now it's time to start upgrading all the firmware on your new shiny toy.

Update all the firmware
First step, head on over to https://dell.app.box.com/v/BootableR710 download the latest ISO, copy it over to a USB flash drive with something like Rufus
Once you got that all done, plug it in on any of the USB ports on the server along with a keyboard and a monitor. Once you egt to the Dell loading screen, it should say to press F11 to get to the boot selection screen. Once on there, select the USB drive you have plugged in and and let it do it's thing.
Once it's done, you'll be running the latest firmware for everything on your R710.
(Side note, remember what I said about iDRAC Enterprise, well, here's where it comes in handy. If you can get the IP of the iDRAC without pluggin in a monitor and keyboard (Maybe it was already set to DHCP and your router gave it an IP address) then you can simply remote into the iDRAC, mount the ISO and boot it up. No need for a USB, monitor, keyboard, or anything else. If you can't get the IP for some reason, or don't have the login credentials (Default username:root password:calvin) then you will have to connect a monitor and keyboard to reset the iDRAC settings in the BIOS.)

Install an OS/Hypervisor
OK, now you're really done and are ready to install whatever OS you want. Does it matter what OS you use? Depends on what your needs are. Most of us here run some kind of bare-metal hypervisor (ESXi, Hyper-V, Xenserver, Proxmox, KVM, Didgeridoo (OK, maybe Didgeridoo isn't a hypervisor, but hasn't software naming become ridiculous recently? Seriously! Aviato! How is that a thing!)) Does it matter which one you choose? Homelabbing is mostly about learning, there's really no wrong answer as long as your learning. If you're looking to get a specific job with your new skills, look to see what the job requires. Already using something at your current job? Use that, or try something new. ¯\(ツ)/¯

Final thoughts
So I think I got most of the major topics that come up here often. If you think of anything that needs to be added, something I got wrong, or have a question, PM me or just post here, our community is here to help.