Theres a million suppliments out there that can help you out or that you can just waste your money on. But theres a few that are most important and have been tried and proven:
-Eat! Get enough healthy calories, if you don't eat enough you'll never get great gains even with the best suppliments.
-Whey protein
-Multivitamin
-Creatine monohydrate-> personally I don't take creatine tho.
Basic summary of the suppliments: Your muscles need protein to grow and repair, thats why people take protein shakes after working out, to supply your body with enough to make the repairs. The multivitamins besides just being good for you, also help in muscle repair and energy etc. Creatine helps your muscles retain water and increases the energy available to the muscle, you'll get some gains in strength and size while you are on it. Just make sure you drink enough water if you plan on taking it.