Sit ups don't help reduce belly fat. They train your abs, which you won't see because of the fat. Also, there are much better core exercises you can do to train abs/oblique etc.

The three rules to lose that fat, in order of priority;

1. Diet - count your calories, get rid of processed foods, high protein
2. Strength / Resistance training - you burn a shit load of calories at the gym building muscle
3. Cardio HiiT training 25mins for times a week to speed up the process when you get down to a decent body fat percentage

Cardio is a tool put on top of the foundation of the first two when the process slows down a bit and you need to push the body harder to get down into the -15% body fat range. You shouldn't rely on cardio to lose belly fat.