What is the best thing to eat to get a flat tummy?

Flat abdomen. To get a flat abdomen, it matters more that you eat less, than what you eat. The types of food you eat are most important only in how many calories they contain. So, eat smaller portions and eat mostly high fiber or low fat foods such as vegetables, avoiding sugary, starchy, or fatty foods.
Diet + exercise. There is no single food you can eat that will give you a flat tummy - but eating a healthy diet, achieving a low total body fat, plus exercises such as abdominal crunches will help you get there.