Cardio, cardio, and more cardio is what i would suggest for you! Elliptical, cycling, shuttle running on the treadmill, all these basic cardio exercises would give you results in a matter of few months. But, if you want a rigorous workout that's going to help you tone your stomach muscles as well, then abdomen exercises are the best. These are the natural methods. Apart from that, start making a list of calories you consume per day (Yes! Literally). Consume food that has low calories. Eat the bare minimum that would help you live your normal life for the next 4 hours, and then go and eat some more, spacing your daily food habits like this would prevent fat from accumulating anywhere in your body, especially your belly.