I get this question a lot... I believe the most difficult part is trying to figure out what type of rider you are and what type of riding you want to do... Freeride; Downhill; DJ/Park; XC; Singlespeed; Fixed gear, etc...
After you figure out what type of riding you want to do. Then what I tell EVERYBODY is, go to you LBS and talk to the guys/girls there. They have the best resources on fitting you up with a bike of your choice.