Lift free weights. Lift as much weight as you can handle keeping good form while doing 3 to 5 reps per set for about 5 sets. The exercises you do, in my opinion, should be bench presses, squats, and bicep curls -- all with a straight bar. Most of the other free-weight exercises are not for beginners who wish to gain muscle mass. Eat plenty of protein and give yourself time to rest and heal up. That's when you gain the mass. If you are a teen, you will probably not gain as well as an adult male, but don't give up.