Fat is an energy source for the body, made available to feed the body when other sources of energy are not available. The solution to getting rid of belly fat (and indeed bodyfat from anywhere on the body, since energy is taken out of body fat globally rather than in isolation), therefore, is to make your body need to use fat as an energy source, which is achieved by expending more calories (calories are a measurement of energy) than your consume.
The maths side of things is to figure out how many calories you're consuming daily (easiest way to do this is to measure everything that goes into your mouth and multiply the weight by the nutritional information) and then figure out a way to cull 200-500kcal/day without losing the nutrients you need (and nutrients include macro-nutrients, which is protein, fat and carbohydrates; and micro-nutrients, which is vitamins and minerals). Often this can be achieved simply by reducing soft drink, chips, cookies, desserts and other party foods, but if you're already eating a balanced diet, it can get a bit trickier.
Increasing physical activity may or may not benefit you. As a member of a fitness forum, and as a qualified personal trainer, I know I'm supposed to tell you that exercise is great -- and it is -- but an increase in physical activity can be hit or miss for just losing weight. The reason for this is that physical activity, especially intense physical activity, can spur on your appetite, and drive you to eat more, thus balancing out the calories or burned. A lot of people also take their exercise as an excuse to eat more. The gym I used to train in, and that my brother still works out in, has a canteen that serves hot chips and soft drink. My brother is notorious for getting 20min into a work out, and deciding that he should go have chips and soft drink now, consuming far more than he would have if he'd never shown up to the gym in the first place, and defeating any weight loss that might have occurred due to his exercise.
However, if you do get into an exercise routine, an average workout will burn 300-600kcal/hour, and it helps to make sure that you're losing fat specifically, rather than fat and muscle (since, if your body has to lose weight, it will get rid of whatever isn't needed, which will include unused muscle mass). Resistance training is the best form of training for improving body composition. Cardio's not all it's cracked up to be, but it's still okay.