Welcome to the forum!
Start out by reading the guides at this site - http://www.tomshardware.com/forum/288241-31-read-posting-buyer-guides-troubleshooting
Simultaneously, visit some computer shops like MicroCenter, Fry's, CompUSA, and check out the computer cases on display, PSUs, video cards, etc - ask plenty of questions. This will give you a higher confidence level and the satisfaction that YOU designed the system.
Once you have the basic system put together, post it here and ask for opinions.