When I was 14 I weighed somewhere around 140 pounds, the best thing that I would do was get up in the morning, drink a big protien shake with 2 sausage and biscut sandwitches everyday for breakfast. After that go through a good workout(alternating what you do...bench one day, squats the next)and then finish up the day with about 100-150 pushups. This might not be the best thing for you, but I put on about 40/50 pounds of muscle in about 7 months. But then again, your still pretty young so you might have to wait until you get your groth spirt.....

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6'1 195 lbs.

Bench 330 x 1
Squat 425 x 1
Deadlift 450 x 1