First, establish your theme. If you've been assigned a theme, make sure you understand that theme. Find out what scripture says about your theme by reading everything pertinent in the Bible in several different versions. You probably don't need this reminder but...for heaven's sake, do not take verses out of context. That would be a very bad example to set for kids.
Then, break your theme into manageable daily bits. Narrow your focus and develop one point per daily devotion.
Devotionals should be kept short, they are not Doctoral dissertations or Masters theses. These readings are generally quick meaningful pieces that include scripture, an anecdote, and an explanation that expounds on the scripture, tying it in with the anecdote. The ending may offer an appropriate quote and a suggested prayer.
Finally, because we're talking about writing for teens, you've got to know kids. Spend time with young people. Know what they love and what they don't like, how they talk, and what and who is hip in their world today. Use that information sparingly; don't over do it. Unless you are still a teenager yourself, don't try to be one. You probably aren't cool enough to pull that off. Just try to speak the language enough to get kids reading. Let them know that you can relate with them on their level without writing down to them. They'll respect you for being a grown up who cares about them and the Word of God.
I would love to take time to give you more tips, but I've got some devotionals to write. Come back later and check on my progress. I'll work my way through my upcoming camp devos right here.