that I've seen done in a professional health class.
Log everything you eat.
Measure all of the calories.
it's common sense to know which foods to stay away from, and pay attention to all of the things you are eating.
Treat yourself once in a while, but don't go overboard.
Go to an online calorie transferer to see how much you are burning when you are exercising (for running and tennis)
and do the math.
it should be fairly easy,
also dried nuts are very good for athletes and are good sources of protein.
EAT FRUIT AND VEGGIES. ;D