start reading nutrition facts... low fat low carbs.... drink about a gallon of water per day, and go for a jog 2x a day, once in the morning, once at night.... or once when you get home from school once at night, or whatever. Since it seems like you might not be in too good of shape, start off w. baby steps.
foods you might want to eat:
a lot of fruits and veggies(they have frozen fruit bags at wally world, theyre the shit)
salads w. chicken or steak, w. little or no dressing, or low fat dressing
lots of water(STAY AWAY FROM ALL SOFT DRINKS!)
fruit juices such as orange juice, apple juice, etc
eggs are always good
try low fat sandwich meats(no mayo!) try mustard, or any other low fat dressing
just think about what youre eating, i know its real easy to eat fast food, but with drinking a lot of water, your hunger will stay down a lot aswell