If you have a bad back, I'd be very loathe to recommend exercise machines. How about learning to swim? There are lots of people at our local pool who go there to learn.
If you don't fancy that, this might help - and it works.
Pull yourself up to your full height, pull your shoulders back and relax them down, put your hands on your hips and push down, at the same time consciously stretching your body upwards and tightening your stomach muscles. Hold for 5 seconds, and release. Repeat this five times, several times every day, and it will help at lot.
There are also exercises on the internet, like these.
http://www.allaboutyou.com/diet-wellbeing/flat -stomach-exercises/v1
Don't forget, posture is important. If you stand straight (breathe in and pull yourself right up from your stomach, let the breathe go, but maintain the posture - and do this whenever you remember to), you'll feel your muscles tighten, and they will improve. It's all about using them really. Good luck.