Decide if you want to buy or build
If you want to build search on google for computer building advice then search for parts on newegg.com, tigerdircet.com and slickdeals.net.
If you want to be lazy just search around slickdeals.net for some computer sale ($4-500) that has a 350W powersupply and slap a nice videocard in there, you'll have a computer capable of playing new releases for the next couple years.