The biggest decisions in your computer are going to be which graphics card, CPU, and power supply you want.
These two websites have Video Card and CPU comparison charts, respectively.
http://www.videocardbenchmark.net/
http://www.cpubenchmark.net/
Make sure to get a good PSU that has enough power to support your graphics card and CPU. I have had the same PSU through 2 builds because I bought a good one, and it'll likely last me through another. You don't want a crappy one that could fail on you, for obvious reasons - cheap ones sometimes do.
Then all you have to do is:
- Buy memory - get 16 GB imo, no more is really needed (it's mostly similar, I just choose whichever ones are best reviewed on Newegg)
- Buy a hard drive. Get an SSD. They're pricey, but absolutely worth it, especially for gaming. I pressured my dad into getting one for his build and all he can talk about is how amazing his computer is and how fast it is. He loves the SSD.
- Buy a case
- Buy a monitor
- Acquire/reuse keyboard/mouse
Good luck my man.