how do i stay in shape? without going on a jenny craig or weight watcher thing?

i hate it! i have unwanted fat… (btw: this is my sis writing this so im not actually having these probs she is) i am 12 years old and i got a belly and i hjate it idk how to fix any of this… i love to eat but i eat healthy i eat fruit and milk and water i do evberything possible what should i do?