In fact, the best is running and abdominal aerobic exercise.

Jog 1.5km or more, and then do the action of abdominal ripper (there is a video on the internet). You don't need that much strength at first, but each group can be 10. When you feel free, you can gradually increase the number of each group.

Simply doing sit-ups can also be thin belly, but the effect is not as effective as the above methods.