Before choosing one of the methods, consult with your doctor. When my friend was at your age, she started putting up weight veeeery fast. It turned out that there were some changes in her harmones (it is absolutely natural at your age) and that was the reason. The doctor prescribed her some pills and she started losing her weight. So don't worry. First - consult with a doctor! Good luck!!!
For you to lose weight, set a goal. Then, you can start a regular exercise program. Eat healthy fats from nuts, seeds as well as olive oil and high fiber foods like vegetable salads. Read more on http://www.ehow.com/how_4622_lose-weight.html.