Trying to get a teenager to do anything they did not think of themselves is hard. It is even harder to get them to work out and stay in shape. If the kid is not actively involved in a sport it can be next to impossible.
This may sound funny but if your idea of exercise is getting up for new batteries for the remote control you may not have a lot of room to talk. You need to set a good example buy working out yourself. Working out is not the only thing you can do. Serving well-balanced meals with proper portion sizes well help. Over eating and not having healthy food has put a lot of teenagers in the overweight to obese side of the scale. Start by looking at what you eat. If you supper size everything or eat out a lot start cutting back the amount you are eating. Remember that even what you are drinking can add to your weight problem.
If you look around you will find 24-hour gyms popping up all over the place. They have made it easy to work out. You can go when it is convenient for you. You just need to force yourself to get off the couch and take the kids with you. Get outside and walk around. The fresh air will do you some good.
The best thing you could do is start taking care of yourself. If for nothing else it will help you feel better. If you make fitness an important part of your life, the kids will see it and will start doing it for themselves.