As has been said a thousand times, build it yourself, its cheaper, ect, ect, ect.
Because seriously, you're going to pay a fortune for a store bought gaming PC, and it'll come with a bunch of stuff you don't need to inflate the price tag normally - 16Gb RAM, a 980GTX and a high-end i7 processor.
For most games, you'll need a mid-high range i5 at most, 4 generations back and 3 models down for the graphics card provides adequate performance, and you're not likely to need 16Gb of RAM whilst we're still trying to make the transition to 64 bit for most games [32 bit programs can only access 2Gb of memory].
PCs are fairly easy to build, however I can understand being uncomfortable with it, so getting your friends to help would be a good idea. But, if you want to do it yourself, look up some Youtube videos and follow them. Its pretty easy, just don't wear anything that's likely to generate a high static charge, or touch the parts to anything likely to have a high static charge. Grounding yourself before working on the PC is a good safety measure of sorts, but isn't 100% necessary - you get away with not doing it most of the time, depending on what it is you're fiddling around with.
If you must got for a pre built... You're going to want to run it past your friends or us prior to buying, just because of how the pre-built market is. You could pay a fortune for a pile of uselessness, or could pay very little for something even more useless. Odds are you'll be looking at between $800 and $1200 if buying pre-built though.
In terms of mods... Its open country. How do mods work? Depends on the game.
There are two main types of mods. You've got the Dark Souls fix mods, or the KOTOR 2 restored mods, or the alternate Mass Effect 3 ending mods, where the mod makers know the game engine to some extent, know some programming, and program up some plugins to slightly alter how the game runs, adding in missions and the such - or in some lucky cases where we get the source code, actually editing the game itself. These are rare, however, and most games have no mods at all because the community isn't passionate enough to do this, and this is required to mod those games.
Then you've got your Warcraft/Starcraft/Skyrim/Sins of a Solar Empire/ect. Mods, where the game designers have designed the game in a fashion such that it is meant to be modded, and you can launch a dev-made editor to mess around with stuff in game, save it as a file, and have the game load that file and use its changes. Games like this are rare, but in the ones that exist mods are really common.
Installing them happens in one of two ways normally. If its the first type of mod I described, it'll generally come with an installer that will install all the files for you, if it finds the game installed on your system. Makes installation easy, as you do nothing. If its the latter, you copy and paste the mod into the 'mods' or 'maps' folder of the game, and its installed - you just have to choose to run the mod at game or map launch.
There are a couple of exceptions, like Minecraft where you have to open the main file and drop some folders in there, but if its any more difficult than saving the file to your desktop, the mod maker will generally give instructions on how to install.
Communities for mod making are pretty damn open really. The first mods I described don't really have a huge creator community. You have a few creators, and a bunch of fans.
The second kind I mentioned has a lot of creators and a lot of fans, and anyone can make a mod and put it up on the internet for others to use. Do be prepared to be heavily critiqued if you do this though, as there'll be a lot of people posting trash, and only a few people posting really good stuff.
There are also a lot of communities that exist around teaching people how to mod the sorts of games where the developer provides tools for you to mod with. Whilst anyone can make a mod, there's a lot of things you need to know how to do to make a good mod, and that's what these sites will try to teach you.
But, anyway, welcome to the Glorious PC Gaming Master Race. Once you get your first PC set up, its really not that hard to do anything these days. Shove a disk in, install the game if you have the disk space to do so, and then run it. Maybe update drivers once a year or so. Its come a long way from the good old days when things really were complicated and compatibility was almost non existent.
