Hi, OP --
Welcome to the wonderful world of Sunday School teaching, where you will make new friends (of all ages) and challenge, renew and and go deeper in your faith! Plus, you're now in the same club as Jimmy Carter, Stephen Colbert and Anne Lamott -- how cool is that?
I've taught Sunday School in an Episcopal church for grades K-8, and, as you've probably guessed, I love it. It's definitely the best church lady gig I've had. From this experience, as well as my perspective as the parent of young people in their teens and early 20s, I'd suggest that you emphasize the following themes:
God is love. We're all broken; God knows it and loves us anyway.
Be kind and loving toward others in your intentions and actions -- even when they're mean or petty. Everyone is broken; everyone is fighting a difficult battle (yes, I know this is attributed to Plato, but it's a beautiful and succinct statement of Christianity as well).
Pray -- I have found that kids have really interesting and original ways of praying, and are surprisingly comfortable talking about prayer.
Everyone has doubts -- yes, even your Sunday School teachers, but we still show up on Sundays and try to do our best to grapple with them. God is totally OK with this.
This church and congregation are here for you. There are adults here who care about you, and you can have friends here who love you for who you are -- not for where you are on the popularity food chain at school.
As Christians we are called to create God's kingdom here on earth -- to love and serve one another and to care for God's creation. Kids love to do service projects -- look for every opportunity you can to work with them in service. Acts will nurture faith.
Hope this is helpful, and that you have a great time with your class!