With such a topic so broad we truly try to cover the basics from all angles in this group. Nothing too big or too small. Nutrition is as significant if not more as exercise is to reaching your goals so learn all you can.
You should think carefully about doing sit-ups regardless of whether you have scoliosis or not. Sit-ups but strain on the lower back where as crunches just exercise the abdominal muscles. Also you will probably never lose your tummy by just doing abs exercises, you need to include more general cardio work in your routine (and of course a controlled diet).
When you lose fat it dose not matter which muscle group you exercise the loss happens all over (unlike when building muscle). So using only a small muscle group (such as the abs) will not produce much fat loss as they tire fast, where as using a large muscle group (such as your legs) will produce much greater fat loss.
I am not sure if there are any limitations placed on your choice of cardio work because of your scoliosis, try asking that question (here or to your doc) or doing a web search. Ab specific exercises will tighten the underlying muscle but do little to reduce the fat layer over them (your tummy). Also you might find better advice can be given if you give a few details in your profile, anyway hope this helps.