In your position, I would consult a doctor before starting any exercise program - they may be able to help you come up with an exercise program that will work for you.
An important part of sustainable weight loss is eating sensibly - you could start by examining your diet and trying to choose lower fat / sugar foods or reduce your intake of more unhealthy foods.
There are lots of exercises you can do in your chair that involve lifting / extending your legs / arms, etc - anything you can do that means you are moving around more is a good start, but if you are concerned then it is best to speak to your doctor.