Your doctor can refer you to a good physiotherapist in your area. also, while you may not be able to walk for long distances, there are exercises you can still do. Swimming and water aerobics are two very good options. Being in the water will help take stress off your joints. Injured athletes routinely hit the pool for rehab.
Also, you could sit in a chair and do upper body strength training exercises with hand weights or resistance bands. There are even cardiovascular workouts that can be done while sitting in a chair too.
Have you read any of member INDYGIRLs blogs ? If not, I'm going to link you to her page. At her highest weight, she weighed over 460+ pounds. two years later and she's lost half her weight. She's written several great blogs on exercising with disabilities. She used an arm peddler for exercise at one point. that might be something you could try too.
Mostly, you could talk to your doctor to get clearance. when is the last time you had a full physical ? A healthy lifestyle should always start with a trip to the doctor to get the vital signs checked. You should know your current blood pressure, blood sugar, cholesterol, etc... all of these numbers will improve as you become healthier. You also need to know if you have diabetes or not. If you're diabetic, that will effect what you eat.
Here are a couple of links to INDYGIRLs blogs. read them. they're really very good. www.dailyspark.com/blog.asp?post=4_workout