what is a good way to get a flat stomach?

what would be a good way for you to get a flat stomach, other than dieting.

i’m 13[girl], and want to have a flat stomach for the summer.
anybody know a good way for that to happen?
please?
and serious answers only please.
[ps, what are leg curls? and if you suggest leg curls and sit ups, how many per day would you reccomend?]
please, can i have some more answers.
i need to start whatever people suggest asap.
please?