How can I become a better 200 sprinter in track?

In the fall running Cross Country would be best. Talk to the track coach to see if there are any groups doing conditioning. Don't be overly worried about a strict diet. You are still growing. Just don't eat too much junk food, or overdo it during the Holidays. You got the same pair of legs to push you around the track, so the heavier you are the more they will have to work for you to go the same speed. But too light and you may not have the strength or stamina to finish. For Cardio a good run at least once or twice a week may keep you where you are at. More often will help you improve. But don't for get about fun aerobic workouts like dance.