The answer to your question depends entirely on "how fast" you want to lose the weight and how much weight you want to lose...
Safe weight loss is considered to be 1-2 lbs/week, using a healthy diet and moderate exercise program. Since you are only 15, your body may not be fully developed, therefore, alternative weight loss programs may not be safe for you.
You should talk to your parents/guardians about making sure there is plenty of healthy food for meal and snack times so you can eliminate as much processed food as possible.
Exercise can involve anything from a brisk walk to dancing, swimming, biking, etc. You should aim for at least 30 minutes/day, but if you can't in that much all at once, try for 10 minute increments, 3-4 times/day...