First of all, I applaud you for beginning an exercise program. You have taken the first step towards losing weight and living a healthier life. The answer to your question is tough to know since I don't know anything about your other medical conditions and any joint or muscle problems you may have. My gut instinct tells me that you should start with something simple that will burn a good amount of calories, but will not dissuade you from continuing to exercise.
One exercise people like to start off with is walking. A brisk walk for 2-3 miles per day (which should take at least 45 minutes) can burn a higher percentage of fat than more vigorous exercises. Another great exercise is aerobics. This type of exercise can burn lots of calories as well, and may work more muscles than just walking. The trouble is that you will either have to go to the gym or buy a video. Probably the best exercise you could do is light swimming. This takes some dedication since you need to have access to a pool.
This is a great question for you to pose to your primary care physician
. He or she will be able to recommend an exercise program that will be both healthy for you and simple enough to continue for an extended period of time. Good luck.