Honestly, the single most helpful thing someone of your age can do this summer is to get enough sleep. Go to bed "today" (before midnight) every single night, weekends included. Consciously aim to get up at about the same time every day (again, weekends included), and to make each night's total sleep about 45min more than you're used to. Get away from the TV/computer not later than an hour before bedtime--those screens and their light have a stimulating effect that reduces the quality of your sleep.
And you're going to need that sleep anyway, because my other recommendation is that you find the Summer of Malmo (use this site's Search function, or Google) and follow it. Adapt it for your personal situation--don't make crazy jumps in weekly mileage, but rather build up gradually and steadily from wherever you are at present--but keep in mind that successful training is a *combination* of stress AND RECOVERY.
It's been wisely observed that "a year of growth beats two years of coaching," and for no one is that more true than the typical 14- or 15-year old male. And almost all your growth (which includes a lot more than just getting taller) takes place when you're asleep.
Good luck. Please keep us posted (maybe by bumping this thread periodically) on how your summer is going.