Need To Gain Muscle and NOT Fat

Like rtalencar85 said, LIFT!

But you should also try supplementing Creatine, there's a good guide on Creatine use at: http://www.thecreatinereview.com .

Make sure you keep yourself well hydrated while you're on it, but you should stay hydrated regardless.

I've definitely noticed an increased in lean muscle mass since starting creatine.

Good Luck!