Welcome to the board mate!
At your age, if you feel the need to take something, try this...

Protein Powder
BCAA's
Creatine
Glutamine
Arginine
Multiple Vitamin
Fish Oil

I know hearing advice from the experienced members here, telling you 21 is too young for AAS is a bitch. They are not telling you this because they are wanting you to be hating on them. They are doing this because your body is still developing, and i'm referring to your hormonal system. It will continue to develop well into your mid 20's. Taking steroids now will shut you down at a time your body needs to produce it's own natural test to finish developing.

Here are some things you may want to read if you have the time....

http://forums.steroid.com/showthread...T-use-AAS-imho....

and this...

http://forums.steroid.com/showthread...g-and-Steroids

http://forums.steroid.com/showthread...e-(Great-Read)

alot of youngsters come to this board thinking that we are pro steroids under any circumstances. This is not true. We are pro steroids and their intelligent use. Because we feel strongly that youngsters should NOT begin AAS until their mid 20's, this is why you will have difficulty getting advice on this subject matter from us. No one wants to be responsible for giving bad advice. Make sense?

Until then, try to focus on diet and exercise as others have suggested.

If you need help getting those two going, we have great resources in the various sections off the main directory.

Good luck!
---Roman