How can I gain muscle fast?

Hello, I'm nearly 15, i'm 5 foot 11 inches and weigh about 9 stone (57kg) and would really like to get more muscular. The only sport I really play is tennis, so I have very strong arms in comparison to the rest of my body and I'm naturally quite skinny so I'm not fat. Also because I play tennis my right pec is bigger than my left one so any ideas how I could even it out? I would really like biggers pecs and I wouldn't mind a six pack either. Could anyone tell me how to gain muscle fast? What should I be eating? What I shouldn't be eating? What exercises I should do? And apparently you can cause stunted growth if your not careful so how can I avoid that?