I have a lot of health issues, and my fibromyalgia, osteoarthritis, and autoimmune issues are the worst (rheumatoid arthritis hasn't been ruled out, but hasn't been confirmed. Whatever autoimmune disease I do have, it seems to be connective tissue related - the cartilage of my nasal septum, scarring on the lungs, and possibly the joints).
At any rate, pain and fatigue make exercising and wanting to exercise difficult, and yet gentle exercise also improve the pain and fatigue.
I started with warm water exercise. If you can find a local warm water program, or any indoor pool which is kept at 85 degrees or warmer, it's amazing.
The Arthritis Foundation, or the local department on disability and aging resources, or United Way... these organizations should be able to tell you if there's a certified warm water exercise program in your area.
Our local program has had to raise their rates so high that we recently made the decision to join our local YMCA. The water is about 82-84 degrees which I can tolerate pretty well, but the 90 degrees of the warm water pool was amazing. There was no "adujstment period" to the water at all.
In the cooler water, my joints hurt until I've been moving for a few minutes. Using the hot tub afterward is my incentive for going.
I've also been using the treadmill and the recumbent elliptical.
I keep a little log book, and that actually has helped with motivation, because I write down the date, what I did, and for how long every time I go. I'd try to meet or exceed what I did the previous visit (but a little bit, not a lot. One or two more lengths of the pool, a minute or two on the treadmill...)
The important thing is not to get discouraged or angry at yourself when you don't succeed quite as well as you'd hoped. Consistency is much better than intensity. If you hurt yourself, even just a little bit, it can make it harder to want to go back again.