1. First, I would try to figure out if there's anything else going on with them to see if there's a reason they aren't motivated. Are they self-conscious? Anxious? Afraid? If there is, perhaps, something bothering them, also make sure to help with that.
2. Make sure you aren't bombarding them. Try sitting them down and kindly telling them that you are trying to help them become better. Ask if there's anything that you can do to help motivate them to be more active, or why they don't want to be motivated in the first place. Even if they act stubborn, always be kind and patient and state your opinion with facts in a way that won't make them feel bad.
3. Perhaps try and find some fun activities that are active that they can try out. Even if it costs an extra penny, if your teen likes the activity and it's healthy, then go for it! Maybe they'll enjoy golfing. Maybe they'll enjoy exercise-based video games. Maybe they'll like dancing. There's so many option on how to be fit and active, so try helping your child explore these.
4. Perhaps try being a more obvious example to your child. If you want to try an approach of doing yoga, going to the gym, or just eating healthy, make sure you start out doing it together and making it a fun bonding activity of some sort. After a while, though, if your child begins enjoying being more active, give them a bit of space so then they feel independent and like their activeness is also due to themselves!
I hope this will be of help to you. Good luck :)