The fundamentals of bodybuilding involve more than lifting weights. Nutrition is 95% of building mass. For example, consider
the automobile: The engine is only a tool that puts the fuel to work. Without fuel, the engine doesn't run and the car won't
move, but open the throttle to permit more fuel into the engine, and you can make your car go so fast that you'll end up in
jail for speeding.
Your body is analogous to that car. Weightlifting is the tool (engine) that converts food (fuel) into muscle growth (acceleration). None of the fundamentals is more important than food.
That's not an easy concept to accept. After all, training is a part-time thing. You go into the gym, lift your weights and, after a couple of hours, forget about it for the next two days.
Your diet, on the other hand, is a responsibility that haunts you through all your waking hours. From the moment you get up in the morning to the moment you go to bed at night, you have to be cognizant not only of what you're putting in your mouth, but of what you're not putting in. Not only do you have to eat enough, but you have to eat the proper foods and avoid those that are not conducive to muscle growth and good health. At the same time, you have to think about your homework, varsity practice, band rehearsal, family, girlfriend, friends, community, church and any number of other responsibilities and distractions.
Admittedly, that sounds like a heavy load, but you're a big boy now and can handle all of it, so buckle down and learn the basics of a good bodybuilding diet. Once you do, following it will become second nature for you, and you'll do it without thinking about it.
First, make sure you eat foods that are dense in nutrients. The biggest problem I've seen in teenagers is that they either snack on protein-deficient junk food or simply don't eat enough. In either case, you won't build muscle tissue. What you need is adequate calories in the form of healthful carbohydrates to supply your energy needs for the day. You also need enough of a protein surplus not only to rejuvenate your muscle tissues, but to make them grow even more.
I wish someone would have set me on the right path when I was a teenager, instead of leaving me to find my own way. Lack of diet know-how probably postponed my success a good five years, because learning what's necessary to become a top bodybuilder takes lots of study and research. Since I've been through it, though, maybe I can save you some time with the following tips.