What should I do to lose 20 pounds?

What should I do to lose 20 pounds in 3 to 4 weeks or the end of August? I would really like to be around 140 lbs. (the latest is September) in at latest 2 months! But I need help with knowing what to eat, drink, and how much to exercise. Thanks! BTW I am a very picky eater! So please give me some great and tasty recipes. Also I am only eleven so I need something that I can do by myself and easy to do. Thanks!