Everything Carleton says.
And since you ask about diet, the same applies to a junior as to any serious athlete - eat a healthy, balanced diet, and especially as you are still growing stay away from strange supplements as long as possible (leave out the creatine etc etc that a gym environment might try to get you into).
How do you know its healthy and balanced, without getting into too much detail? - if you put all the food you eat in one day onto one plate, approximately a third of the plate should be taken up by fruit and veg, a third, protein, and a third carbohydrates (potatoes, whole grains, etc, overly processed starch like white bread, crisps etc, is not ideal). If you were riding endurance rather than sprints mainly, you would up the carbs a bit. fat is also important, but not in the form of fry-ups, greasy chips and fastfood format.
In terms of supplements, a good quality multivitamin is great, and all that is really necessary until you get into really heavy training.
Its cheesy to say, but I'm going to say it since we all 'know it' and few of us apply it: your body is only as good as the fuel you give it. Junk food and an imbalanced diet wont help you, and will eventually hold you back. Even if you can 'afford' to eat it from a calorie perspective.