Well do you understand how to build a computer ? That would be the first step to find out how you would put it together :]
Main things are that you want a video card that is 256 mb or more. Without a nice video card, gaming will suck on a computer.
ATI or Nvidia are good.
You want to have a fast processor, I would wait for the new Intel processor that's coming out :] I built mine with an Intel and I'm use to it. Even though AMD Is better for gaming the new processor from Intel might just change that.
1GB of memory is good.
As far as a hard drive goes you will want a larger one. Games today keep getting bigger and bigger. So you will want 100+ GB hard drive.
A good place to buy from is www.newegg.com