I agree swimming would probably be the best type of exercise - at least to start. It is very easy on your joints and back. You don't necessarily have to take a formal class, just getting into the pool and swimming wiould be good. Of course, classes would be more intense and might help to keep you motivated having others there with you, but if you can't find a class or afford one, just the swimming alone would be beneficial.
As stated in the above post, do be sure and check with your doctor before beginning ANY type exercise program.
Honestly I think you should look into swimming exercises. You should talk to the instructor and your doctor before trying a class like water aerobics, but that low impact environment I think is really ideal for you.