Good diet & exercise is key. Exercise to consist of both weigth training & cardio.
There are some supplements that can be of use to help burn fat/give motivation. They usually consist of ephedrine, caffeine, & aspirin. Eph is on the market again and can be bought legally, but pseudo-ephedrine can be substituted. Do some forum-trolling to find out the proper mix if you want to mix your own.
Again, diet & exercise is key. There is a "curve" invloved, however. Once you get ahead of the curve, dieting & exercise is easier and more effective per unit time spent. Getting ahead of the curve can be a real pain. This is where the supplements come in handy to give you a boost ahead of the curve.