M.T.
I would start with lying on the floor and doing a few crunches... without bringing your body up to actually do a sit up. That would be too early yet. Strengthen it first. This will also help your abs. But that is how I would start and then I would cont to surf the net for other exercises. Good long walks also strengthen you back along with every other muscles. Lying on your back and doing leg lifts about 6 to 10 inches off the floor is another good one. Lay on your stomach and lift your leg up, one at a time and then facing down on hands and toes, lift your leg back/up toward the ceiling, one leg at a time. Those will help for starters.