Hi.I'am 12 years old, my problem is: I feel like my boobs are big, and I hate them (especially)when I was dancing and running in my school

You can't make them smaller, but you can talk about this with your mother and have her take you for a good bra. That can minimize the bouncing.

If you are having issues it does sound like you are not wearing the proper bra size, You may want to see about being fitted for one to ensure the right fit.

Also for running, a good sports bra ( not all bras are made for the same thing)

I have the same problem! I'm 12 too, and omg I can't describe how much I hate it. All of my friends are flat chested and I feel so embarrassed. I'm trying to find a way to make them smaller too. Sometimes that's where the carbs from junk food goes to your boobs. I hate eating healthy, as funny as it sounds, but you kind of have to bare with me here. Just keep up your exercise more then you already are.

Get a sports bra for running and get a regular bra with no padding and just be proud of them. Ur a woman, your supposed to have them. (tht doesn't mean wear a bunch of low cut shirts and hang them out, but don't feel like you have to hide yourself or wear huge clothes, if the flat chested can work and fitting Aeropostale shirt, so can u)