How can I get a flat tummy?

I can do 30min to an hour a day. But, I need some program or something telling me what to do. Can you help? I am 14 and I weigh123 pounds. I like my weight okay, I just want a flat tummy. Both my sisters do. One was born and will always be that way the other stopped eating for awhile. She’s good now. But, I don’t want to do it that way. I want a healthy way to do it. Some kind of workout or something.