On my website, (fulzen.org), there's a free trainer, under the "coaching" tab, you just have to create a log in so i can track how many people are using it. There are also a lot of motivational/inspirational articles, and then cheap supplements too (:
The good exercise for teens is participate in various outdoor games such as Cricket, Football, Volleyball etc. These games maintains your body as well as keeps you healthy. And the most important thing is that you didn't get bored while playing.