I don’t give alot of adivce on this site, but I feel like I was in your shoes not too long ago and made every dumb mistake I can think of.
When I was about 15 I jumped into training, dieting and lifting like a madman. I got in on one of those free trial offers from Flex magazine and I was hooked. I bought all of it hook line and sinker. Pretty much my whole life was absorbed by training for several years. While this did take my mind off of some issues at home, it wasn’t very productive.
So with that in mind, I suggest first and foremost: be a kid. Don’t worry about counting out everything you eat or logging down every detail of your training. Have fun, don’t stress out about it and have fun some more.
Train the whole body. Pick a program and train the whole body when you train. Do squats, deadlifts, pull ups/chin ups and overhead presses. Stay away from machines. Learn to perform the olympic lifts if you can.
Don’t believe all the junk from muscle magazines about aerobic exercise stunting your gains. You’re 15, run, jump, play sports and grow. Don’t worry about it. When you’re a competitive strength athlete you can carefully plan out your other physical activities to maximize your gains.
Eat. Eat. Eat. Eat. Eat. Don’t get too caught up in dieting. You’re 15. Eat. Eat everything your parents feed you, and maybe do a serving a day of Grow!.
Stay away from expensive supplements. You don’t need them to make gains.
Steroids are a tool. Nothing more. There are no more good or bad than a hammer, though they are a more exacting tool. Like an automobile, if you are careful, informed and not an ass, you can use them both safely.
You don’t need steroids at 15. Period. Anyone in high school who is using steroids is an idiot. (I hope you teen juicers are out there reading this and take offense at it, because you are.)
Hope all that helps.