Ok, I have answered this question before, but apparently nobody likes to listen. There are two ways to have a big belly. The first of which is by being fat. The only way to get rid of that is by reducing your body fat. You said this is not the case so that leaves the second; distension. Distension occurs when the muscles of the abdomen are too weak to contain your internal organs, and they push outwards. (nice huh?) The way to get rid of this is by strengthening those muscles. CONTRARY to popular belief, situps and crunches are NOT the best way to do this. This range of motion works the rectus abdominus, or the muscles that make up a "six pack" These muscles aren't really responsible for reigning in the organs. What you need to do is exercise your transvers abdominus, which acts like a natural girdle. TO do this, you should either stand up straight, or get on your hands and knees (doesn't really matter as long as your back is straight), and try to suck your bellybutton through to your spine. I would recommend doing this for 10 sets of 10 second holds starting 2 times per week. If you are distended, this means that this muscle is weak, and you don't want to over-do it to start, a tear here is NOT pleasant. After a few weeks of this, increase as you like, and you feel comfortable with.
EDIT: sorry, seems that you said you DO have some belly fat. Only to get rid of this is by decreasing your overall fat composition. You can NOT spot burn fat. Loads of situps will not get rid of the flab directly over the stomach. I recommend a sensible diet combined with a moderate exercise routine. What do you like to do? Jog? ride a bike? play tennis? I would also recommend some moderate weight training in your routine. And don't worry about getting bulky as a lot of girls do. You simply don't have the requisite hormones to get big and bulky, so it really isn't an issue. Good luck.
