Remember your favorite activities from when you were young and try one of those. For most people, walking is an excellent activity to begin with. It has little risk of injury and doesn't require special skills. If you are concerned about your health, consult a physician before beginning to exercise. Begin gradually with short distances and a gradual pace, however exercise frequently.
Every day do something active.
You will be trading the habit of being sedentary and inactive for one of activity. It will take time to change this habit but remember, right from day one you are changing status. No longer a couch potato, you are now an athlete. A step then one more. A walk becomes a journey.