I think if you have blubber then the muscles will develop under the blub and make you look fatter.
You need to lose the blub first with some cardio and diet work. Some people will find it hard to get a pack while others will develop quite quickly, be patient.
I would try three sets of 20 (every other day)and just change them every time e.g normal situps, Then try touching right elbow to left knee and right elbow to left, try with a weight on your chest etc etc.
Crunches are as good and safer than situps though.