Hmm I would pray about it.
Oh! Maybe that's it...you could do topics like prayer and maybe circumstances in life you've gone through and how you grew spiritually through them and remember to give Scriptures...maybe do some on the love of Jesus (I know that may sound simple or not mature enough for teenagers...but the love Jesus has for us is so beyond us..it truly is. And with teenagers constantly looking for love...well, I think it'd be good to do.
)...there's a lot of topics you could do.