I'd encourage you to check out our Senior Health Lifestyle Center. There you'll find some good articles about exercise. Here's a link: www.sparkpeople.com/resource/lifestyle_cen
Make sure you've been cleared by your doctor to exercise, and then you'll just want to go at your own pace. I agree that walking and water exercise are both good choices because they are easy on your joints.
Hope that helps. If you have more questions, let us know.