I agree that being fit and having good muscle mass does increase your self esteem. However I have read some about certain cardio workouts increasing HGH, and putting on too much muscle at a young age will restrict your height. I don't know if there are many good studies on the topic but I will look for some. I started working out with weights when I was young and was built like readrothbard when I was about your age so I understand the desire to lift and put in muscle. But I think at a young age cardio is probably better for you.
There were these twins at my high school - one has a daily dosage of video game while the other has his intake on day-to-day chaotic training. The athletic one is 6 inches taller and is still growing.
There's minimal evidence for such things, but a friend of mine who started puberty when he was eight lifted intensely the month after he began puberty, and he peaked at 6"2, the tallest in his family. He was a committed athlete and trains every single day.
Whenever I'm in the weight rooms, I always see dads giving their little children 15+ pounds of weights along with some mad protein, but years later, they shoot up like any normal person would during adolescence. It's a fascinating sight, but I can assure that it's genetics that play that role.
