You will need to work the muscles, so focus on exercises that use your abs and core. Also try to lose weight by doing cardio. Some good activities are pilates, yoga, dancing, and running
In order to flatten your stomach, you need to make long term dietary adjustments. In addition, regular cardiovascular exercise along with core training is necessary. Results will occur over time.
Lay flat on your back and raise your ankles 6 inches off the ground and hold it there. This will strengthen your abdominal muscles and flatten your stomach.
One key to help flatten your stomach is you should exhale (like the feeling your stomach has when fogging a mirror) and hold your stomach in during your aerobic exercises.