Order a bra online to fit you. Don't buy child bras. I have no clue what they are. Are they training bras? Just buy adult bras, real bras. Figure out your size in those because your not going to stay a child, and your growing up. You're 13. You don't need a "child" bra, whatever those are. Since you said your band is 24, buy a 24 band bra. Now just measure around the fullest part of your bust, subtract that from your band size. The difference will be your cup size. For example, if you measure 30 around your bust, the difference is 6, so you'd be a 24F. :) Good luck.