The short answer is water! If you have been inactive for a long period of time, don't try to run the marathon.
Eat more frequently during the day. Sounds strange. Nope, don't eat more, just more frequently, it keeps your metabolism up.
Not eating, makes your body think that it is starving, and will actually store more fat.
There is really no safe way to dramatically lose weight fast. Depending upon your age, weight and gender 2-lbs per week is considered safe.