This seems like such a simple question but there are any number of answers you may get. This is because there is a lot of misinformation out there. I would like to give you a solid foundation from which to achieve your goals.
First, diets and sit ups will not do anything for you. Diets are designed to fail because it is a short term approach to a long term situation. It is better to change your approach toward nutrition in general. Avoid as many processed foods as possible. They a filled with additives designed to prolong their shelf life and are not what nature intended we eat. Eat more natural things such as eggs, chicken, steak, fruits and salads. My wife lost 25 pounds in a year by just eliminating soft drinks. Do these things and you will be well on your way to reducing your body fat percentage.
The reason sit ups and other stomach specific exercises don’t work is because they are designed to build muscle and not burn fat. It takes 3,500 calories to burn a single pound of fat, nit even an hour on the Stairmaster will do this so don’t bother with sit ups now.
As I said, not even an hour on the Stairmaster will burn a pound of fat. In fact it only burns 1/7th of a pound. To truly energize your body’s fat burning abilities you need to lift weights. This is because muscle weighs more than fat so your body must work harder and, consequently, burn more body fat percentage
However, the types of exercises you do will increase your metabolism for days on end. These are exercises such as renegade rows, dumb bell snatches, clean and jerks and others.