Most methods to lose weight 'fast' aren't very safe or healthy. You are 15 years old, which means your body is still growing and going through many changes. You will need proper nutrients regardless of how much weight you want to lose -- don't do any crash diets. Cut out junk food and junk drinks (ie: things high in sugars, fats, high sodium, etc). Drink plenty of water, and be sure to keep active. If you do this, it will help you a lot more then some silly crash diet.
If you are trying to lose weight then your lifestyle changes will most likely be noticed, but if you are trying to become more healthy, this is something your family should recognize as a good thing.