The first I'll say is this. I have been teaching for a year and have had classes of children, teenagers and adults. Although I was VERY nervous about teaching teenagers to start with, they ended up being my favourite classes! Teenagers can be really fun to teach. They make me laugh a lot because they make a lot of jokes in class.
Some tips from me are:
Show them from the beginning that you will not accept any nonsense from them. Don't appear to be too soft and friendly on the first day. You need to gain their respect.
Pick the right activities. They need to be short, fun and a little bit silly. I've always found great stuff to use with teenagers on this website - www.maryglasgowmagazines.com/teachers
Remember that teenagers get embarrassed easily so try not to put them on the spot or have the class laughing at someone!
I really think you'll be fine once you get started!