I too am 21 myself, and have done lots of research for myself. If you want to take something safe, the only options would be something non-androgenic, like Anavar, Winstrol and Primobolan. Winstrol is bad for your hair, so i wouldnt reccomend it. Primo is one of the best steroids out there, but it is one of the most expensive too. Var is very safe, but the price is still up there with primo, but it wont get you as big as Primo.
I would deffinatly tell you to increase your calories and you will definalty begin to gain. Diet must be on key, before you hit the gear. I myself have done Anavar with creatine, and seen great gains. They seem to have a synergetic affect.