Heyo, I can definitely help out here. A few things I'd like to know:
1) Can you follow directions? Most computer components these days have fairly standard connections and building one isn't as difficult as it once was. If the manual won't tell you how to put it together, then come and ask and I (and probably many others) can help out.
2) What budget range are you looking at?
3) What are your planned uses for it? (I see DA:O and 3-boxing a 10-year old game). Anything else? How future proof do you want it?
4) Are there any special considerations to make for your computer? (Does it need wifi? Do you need Bluetooth? Do you need an additional sound card?)
With those suggestions in mind, I recommend you review
Bit-Tech's Monthly Buyer's Guide. It has 3-4 different price points and staple builds. I have a few parts that I'd pick up differently, but they are always a great base.
Also, note that you may have to spend extra for a keyboard, mouse, monitor, and Operating System as those are not included in the builds.
You can honestly save 20-30% by building the computer yourself over purchasing a pre-built system from Alienware/Dell/HP/etc.