What're your goals? Other than just gaining mass, what's your intention? Not meaning to be facetious, but do you have intentions of competing professionally? If the answer is no, why use AAS?
You'll have plenty of intelligent people on this board that can guide you through the actual usage if that's your choice, but coming from someone that's just slightly older than you, who had similar intentions at your age, I'd like to gain some insight on your mindset and perhaps offer some advice for your consideration.
Either way, good luck.