Honey, that's what sports bras are for.
If your boobs are the size that they bounce around when you run in gym class, then you need to speak to your parents about getting you some bras. You'll need a sports bra for gym (they are pretty comfortable and offer good support, but aren't as sexy..)
And you'll need to start wearing normal bras to wear around the house and school too.
Don't get too nervous about all this. Every single woman on this planet (pretty much) goes through this, and it's all part of growing up and becoming a hot woman, so enjoy it!
Trust me, every other girl in your class is probably hoping they get big enough boobs to need a bra, so this is a good thing!!
