I appreciate if you can immediately start while young. That way you will learn more on the rise and downfall and will learn with them. If something interests you and you have passion in at least making some write ups for it then it is worth trying. Since teens are fond on accessories, try finding a niche where teens will be allowed by parents buy ( you know, it's either of their mom or dad is going to pay for it ). Good luck.
I mentioned about building a site about off-line businesses or a local news portal before.
How about a site aimed at teens and where they like to hang out?
Review entertainment spots, cafeterias, music stores etc. Also, have a chat room/forum.
Focus on getting the local teen community to frequent the site, then introduce Ads paid for by local businesses and maybe have point cards/vouchers that are exclusive to the site.
The points could be accrued by activity on the site since that helps generate views on the Ads.
Edit: and for the forum, maybe have it invite only to keep out adults masquerading as teens.