Here are some helpful links to follow, whether you know what you're doing or not.

Ultimate Players Association- The official governing body of ultimate in the US
The Ultimate Handbook - A great source for learning throws, strategies, and drills
Ultimate Lingo-The Dictionary of Ultimate Lingo
Disc Shoppe- A large selection of discs (for ulitimate, freestyle, golf, etc.) and other accessories
Gaia Ultimate - Ultimate footwear, discs, and apparel
VC Ultimate - Ultimate footwear, discs, and apparel