As has been said, watch what you eat and exercise. You should also find a group of people who are also trying to lose weight so that you'll have a support group. If you can afford it, Weight Watchers is good. T.O.P.S. is very good and is a lot more affordable.
If you can't spend money on it, look around the web for support sites. Spark People is a good place to start and it's free (they have a teen age division also)!! Find blogs of people who are on a weight loss journey. Start your own blog! Find a Facebook group. That's how to do it without overwhelming yourself. Support, support, support! :-)