[Arcenciel]

Like others have said, using a minimizer bra or a sports bra is probably your best bet. When you get older, if you still really want to make them smaller you can always get a breast reduction. That's not until you're an adult, though.
Another good idea is to just accept yourself for who you are. Breast size doesn't count towards a person's character. Just like how a man's penis size doesn't do anything towards his character. "It's not how big your tool is, it's how you use it." You can love your breasts and learn to be proud of their size. I know I was quite uncomfortable with mine growing big when I was a kid...but now I really like them