What is a good diet & exercise plan to get a flat stomach?

Alright, I’m about 5’1 Im 14 years old and I’m getting chunky. My friends have made it clear to me that I need to slim down. I am a small person, but I have a big stomach now. I need some motavation and a plan to keep me on a good diet and exercise on a regular basis. I don’t eat right. I love to party, so fast food, snacks, and sweets are what I normally would eat. I don’t exercise ever. It’s summer so I figured it’s a good time to get fit. If you can help it’s appreciated. Thanks :)