How does a 13 year old lose weight quickly?

Not recommended. Teens should not go on any type of crash diet. They can be dangerous and you will likely gain the weight back just as quickly. The best way to lose weight is slowly and steadily. Decrease junk food, control your portions, eat more slowly and don't eat when you aren't hungry. Start exercising- work into it gradually and build up. Start with a goal of 1 hour per day at least 3 days per week.