Drink a lot of water . Switching from whole to nonfat or low fat milk is also a good idea, or switching to soy milk is even a better idea. You may find that you don't need to give up calories as much as you need to get off your behind. And don't get stuck in the rut of thinking you have to play a team sport or take an aerobics class - try a variety of activities from hiking to cycling to rowing until you find ones you like. Your goal should be to work up to 30 minutes of aerobic exercise at least 3 to 5 times a week - but it's fine to start out by simply taking a few turns around the block before bed.
Drastic changes are much harder to stick with than small changes.
Try reducing the size of the portions you eat and giving up regular soda for a week. Once you have that down, start gradually introducing healthier foods and exercise into your life. Slowing down can help because it takes about 20 minutes for your brain to recognize how much is in your stomach. Sometimes taking a break before going for seconds can keep you from eating them. Many teens find it's helpful to keep a diary of what they eat and when.