Hi Daisey..
Sit ups are lying flat on the floor and coming up so that you are in a sitting position, but if you have a back problem that wouldn't suit so...Most of us do crunches, which is lying flat on your back with your knees up and bent so your feet touch the floor.. then you need to come up slightly so your shoulders come off the floor as far as you can go, but not so your sitting up.. This is a crunch, you can stop half way and pulse (small forward movements at that height) it and they are also crunches. So you can spread them out over the day, say 50 in the morn and 50 in the eve. Or if you can only manage say 20, then aim to do 25 the next day.. It gets easier and my tummy has tightened up a lot in the 2 weeks we have been doing them. Have a go and remember to hold your tummy in each time you do a sit up or crunch.
Good luck anyway. x