well its not that hard i gained over 5 lbs of muscle in less than a month. then again im kinda psycho and im 18. so anyway i started running 1 mile 3 times a week with 60lbs on. this would be a good idea for you to do too(with less weight) since you are going to be doing alot of running in football anyway. also with that try getting a 5lb weigh and and doing a few hundred reps with each arm. you will get in great shape when august roll around and its rather easy to do. the reps you can do while you are sitting in a chair and the running isnt that hard either. if you live in the city just run to the end of the street or block depending on where you live. but i also recomend that you dont start with alot of weigh all at once. i started right out with the 60lb but im also 200lbs and im built to take the punishment. but since your body is still growing try starting off with 20lb. and if you dont have a vest of something similar use a backpack that is rated to hold a couple dozen pounds.
Work out. Train hard and stay away from fatty foods; I'm about your age and im currently in health, and I would recommend greens and protein shakes and fruit and pay attention to serving sizes and ingredients.