you've got summer conditioning to do right now, im sure if you ask a few buddies, or even your defense coach, he'll give you lots of workouts to do in order for you to get quicker and stronger overall....
in high school, i played running back and middle linebacker, and shortstop/catcher in baseball as well...
honestly, you'll only get quicker if you fully dedicate yourself. i'd suggest lots of squats, calf workouts,
dead lifts, clean presses, and do LOTS of sprinting/running. equal it out. you can sprint all you want, but if you want to start on varsity, you need to be able to last. try to even out the amount of sprinting you do, with running. remember, football is ALOT more mental than physical, and the same goes for training. you've got to be able to push yourself, and your mind, to go that extra mile, and do that last rep, before you can get your body to follow. good luck.