In terms of the how, it comes down to diet and activity. Again, it's important to find out what your child enjoys doing, and don't assume you know the answer. Hopefully they like a sport or activity and then just you can just run with it. Playing 30 minutes of basketball feels like fun and you don't even notice you are exercising and burning calories because you are into the game. Running on a treadmill for 30 minutes however, feels like torture.
One useful trick is to use a diet journal. Have your child write down everything they eat for one week and record how they feel afterwards. This helps you and most importantly them see patterns in their eating habits and how certain foods make them feel. It's no secret that eating healthy foods will give you more energy and boosts mood and eating processed and junk foods make you tired and irritable. However, the journal is an important exercise so your child can see for him or herself.
These are all things that one can do at home without a degree in exercise or nutrition. The internet is also a wealth of knowledge. However, once you start stop seeing results you might want to consider contacting a professional who can take you to the next level. Also, in some cases teens tend to listen to others before their parents. Don't worry this wears off in their twenties.
Above all please keep the conversation positive. You catch more flies with honey!