How can I get a flat stomach in a month? VERY IMPORTANT?

Hey! I’m a 14 year old girl! I live in California! I have to attend this beach part in a month! The problem is that I need a flat stomach by that time! I tried dieting and exercising but the thing is that I don’t know what to eat and wat not to eat! And I don’t know how long to exercise and which exercise I should do! PLEASSSSSSSSSSSE help me!!!