drink lots of water. carry a jug with you if you can get away with it at school.
no more soda or juice for a little while. not forever, just not for a little while.
same thing with processed desserts. anything made with table sugar. again, not forever, just not for a while. you'll find when you eventually go back to eating them that you won't want as much or as often. doesn't really matter because i find grapes and honeydew to be quite satisfying. VERY sweet for the most part.
most important part, go for a walk. doesn't have to be all night, but it should be at least 15-20 minutes for starters. ideally you could do this in the morning, right after school (maybe just take a longer "scenic" route to the bus stop) and once more before dinner. eventually you'll want to step it up as you get more and more in shape, but you'll definately see concrete results in a few weeks, albeit small concrete results.
if it's possible, weigh yourself and get a fat percentage reading (don't know what they're called). maybe the school gym teacher/coach or nurse can help. right them down, and weigh yourself each week, get the body fat thing every 4 weeks or so. also, track the waistline. this will really help with your motivation.
and just remember that the first month is the hardest. get a calender, mark off thirty days, scratch them out one by one. if you can't make it to the end of the first month, you may need some help. usually, when a kid wants to loose weight, mom and/or dad could also stand to loose some poundage. they make great fitness buddies. siblings or even friends can do a lot to help keep you motivated. failing them, you could start a thread here and keep us updated, or at least just me at my email. firstname.lastname@example.org.
sorry for the long post, but i wanted to get enough that at least one or two ideas would get your attention.