It's going to revolve around your diet. mainly.
My advice, don't overkill on the sit-ups, that is not going to give you a flat stomach, all it does is develop the muscles of your core in just extenuates any fat that you have on you sometimes making you look fatter (so your not getting fatter, just muscle pushes it out once developed)
Eat clean, so no harm in eating carbs, should really start high then work way down through out the day, so by end not really eat much if at all any carbs. But try to avoid junk, processed foods, eat regularly as well, small meals, about 5 times a day, keeps metabolism up.
And exercise, make sure that your walking, taking stairs (rather then lifts etc), do the cardio, use some weights etc etc, plenty of people will be able to give better advice then me but that just roughly right i think? anyone who can correct me feel free to cause i'm still learning