First off, how much to you weigh and how tall are you? Also are you male or female? These are much more important than your age.
Don't get panicy about your weight, but eat sensibly. Try and cut out the sweets, snacks and junk food, go for healthy low calorie food and drinks where possible, and eat plenty of fresh fruit and vegetables!! I know...UCK!!!
If you are really concerned about your weight, have a chat with your doctor, but PLEASE don't consider the drastic measures yoy have mentioned. These are very dangerous and will have a damaging affect on you health longterm.
As for excersize, as lizzie (and she's about your age) running on the spot, walking and swimming are all brilliant ways to gently lose those extra calories!!!
Take care and good luck