ORIGINAL: LoneWolf
The single two most important things are diet and training. When you got those down and some experience in the gym, creatine and a multivitamin would be a good place to start. I'm not really a fan of creatine monohydrate since I always got stomach cramps and bloated from it, so I'd say pick up some creatine ethyl ester such as DS Xceed, Gaspari Size-On, BSN Cellmass, CL Magnitude etc.
Good advice^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^