Find a wishing well, throw in a coin, and wait for the good fairy.
You're actually asking the impossible. You can safely lose two or three pounds in the first week of a diet/exercise program, perhaps a little more, but weight loss needs to be at a much lower rate over a long period.
Reduce your food intake, cut out snacks, and exercise (fast walking will do it). You'll be looking to lose around 1 lb a week, but that way you'll be able to keep your weight off.
To lose belly fat in one week, do the right exercises and plan them ahead, a diet plan should also be planned ahead. The will to change should be the main reason for one to gain morale, have a drinking plan and finally seek expert help.