Best exercise for a teen girl? Who is trying to get a flat stomach. I don't know what's the best way?

Flat Stomach. For a flat abdomen eat healthfully ; be physically active. Calories burned must > calories consumed. To tighten ab's do crunches, reverse crunches, exercise ball crunches, pelvic thrusts, lying leg raises, side bridge ; “bicycling” exercise on your back. Engage in regular aerobic exercise (i.e., running, swimming, nordic track, bicycling, kickboxing, walking, cross country skiing, hiking, etc.).
Flat Tummy. Low sugar or carb diets high in protein sources with good fluid hydration is an approach that can help. Limiting your calories is part of this. Avoiding alcohol is also mandatory for that perfect flat abdomen look. You do not want anything that will lead to fluid retention or puffiness.