How can I teach my teen to eat healthy and exercise?

By example. This is easily done by example. A hearty discussion will definitely hel and use living examples to make a point about being unhealthy etc...Being a teen is tough it's all about them being right and you being wrong.
Set an example. Lead by example- if they see parents eating healthier-they might be encouraged to do the same. Bring healthy snacks to movies like fruit, instead of the usual movie fare. Cut out sugar drinks, drink water. Don't keep junk food in the house. Prepare your meals for work and try to do the same for them for school. Eating healthier sometimes involves planning. Show them you are active as well.