Depending on how fat you are. If you're obese or fat, cut all the empty calories (soda soft drinks, ice cream, sweets / candy, cake) and replace with as much healthy salads as you like. Fast pace walking daily for 30mins to 1 hour.
If you're already athletic, that's a much bigger challenge. You basically need to watch the portions in detail and eat healthier versions of each category of food; e.g. brown rice instead of white. You will also need to ramp up your current workout / training regime to make it make it more intense with cross-training as an option.