If you haven't exercised for quite a while, start out by becoming aware of your stomach muscles. This is easily done when you're lying in bed, lie on your back and tighten your abs as if you're trying to pull your belly button down to your backbone. Repeat this and hold it as often as you can.
Stomach crunches are excellent and less arduous/potentially harmful than sit-ups.
When you walk, try to focus on your "core" - strong abdominal muscles will help you to walk better and walking while focusing on working from those muscles will also help to tone them.
