Minimum requirements: A corny keg, co2 tank, regulator, gas in QD, beer out QD, gas and beer lines and a picnic tap.
A better setup would include a shank and a faucet, something like this:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=67135&item=3847914879&rd=1&ssPageName=WDVW
You can also pick up regulators pretty cheap on ebay, one like this would work but you may be able to find one cheaper if you look around:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=11876&item=3848143861&rd=1
Of course you'll also find out quickly that one corny keg is not enough, you can pick them up for around $14 each and where you get them from depends mostly on where you live because shipping them across country can be expensive, let me know where you live and I'll let you know a good place that is fairly close so the shipping charges don't kill you. Most placed that have the kegs also have the QD's as well. You can pick up the gas and beer line at your local hardware store.