Find something active that you really enjoy. I HATE running with a passion, so I forgot about that long ago, but I realized that I really enjoy biking, so that is what I do. Also, getting a dog can help, because they will have to be excercised
Tell yourself that in the future it will help you out. Keep track of the exercises that you do and how long you did them. Keep at it until it becomes part of your routine. Maybe to convince yourself to exercise buy a calendar and if you do something active you can put a check mark on the calendar... Maybe once you get to a certain number of check marks in a row you can go out and buy yourself something that you have been wanting.