The informal way of making money is selling your own stuff. But that's stupid, because then you'll have to buy it back. Maybe you can offer kids who want to learn tennis lessons. Like for me, I play trumpet, and I want to make money by teaching kids how to play...
ask one of your parents if you can get extra money for doing chores, if i'm desparate (not usually cause i getta allowence) they'd give me like 5-10 bucks to wash the dishes