If God has given this responsibility to you, then the first thing for you to realize is that you won't be alone in that classroom. God will be with you! He is more concerned about these young men and women than you are, and He will be present by His Spirit to help you as you teach.
Your work as a teacher should involve at least three steps: preparation, prayer and presentation. Don't skip any of them! First, prepare your lesson thoroughly and carefully - not at the last minute, but all during the week. Base your lesson on the Bible; young people want to know what God says, and the most important thing you can do to prepare is to study God's Word thoroughly. The Bible says, "The unfolding of your words gives light; it gives understanding to the simple" (Psalm 119:130).
Then pray - pray for your students, and pray for yourself, that the Holy Spirit will use you to speak to them about Christ. Satan will do all he can to divert them and close their minds to God's truth - but God can use your prayers to defeat him.
Finally, make your presentation as clear and interesting as possible.
You aren't there to entertain them - but neither are you there to put them to sleep! Summarize your lesson in two or three memorable points.
Encourage discussion also (although don't let it get too far afield). Most of all, point them to Jesus, and urge them to make Him the center and foundation of their lives.