Hey,
I agree with a lot of the other people here who say to have regular exercise and weight training. Eating healthy is great but what I really recommend is watching what you eat! What do you eat? Think about it. Do you eat more of something than another? Do you drink alcohol?
A lot of the time belly fat is either animal fat or beer belly. Do you eat a lot of pork, red meat, refined carbs (white bread/pasta/bagels/cake), high fat cheese or butter? That may be it. If it's not your diet, maybe it's genetics?
Do some simple 30 min. cardio everyday and work your abs only every other day and see the difference! Also this may sound silly but try holding in your stomach as a habit, just by doing this it engages your core. Let me know how it goes. Go on to you tube to find free Abdominal (stomach) exercises.