@T_Bone,
Biking is serious fun... I tried mountain biking a year ago and have been hooked since!
You can buy bikes preassembled, which is the way to go for a beginner. Eventually you'll see nicer pedals, brakes, saddles etc that you'll want to fit depending on what you use your bike for.
I'm well happy with Specialized bikes, they've great after sales support and they frequently score highly in group tests over here. You can find them on
www.specialized.com (free plug). Over in the US, they're very reasonably priced too.
If you just want to ride trails then you'll be looking for a Cross Country (XC) bike, if you want to ride at skate parks and off jumps then you'll need a Freeride bike, which is more robust and finally for the ultra-insane there's Downhill Biking which involves launching yourself down steep inclines. This requires ultra-robust bikes which usually carry a significant weight penalty.
Give it a try... It's addictive!