I had a similar experience as part of my teacher training course at college and I would suggest the following.
1)Have the lesson planned out to the letter so you are confident with what you want to do and can fill in the time slot properly.
2)Try not to think of the "scenario" you are in but try to treat it as just another lesson that just happens to have your peers as part of the class.
3)A sense of humour goes a long way and if something doesn't go to plan, make light of it and try to see the funny side. That will put the class at ease. Remember, the class doesn't know everything that you might talk about and may be learning something new. My particular Lecture was a seminar on film music and by the end of it the class definately felt that they had learnt something new and discovered something that they were'nt aware of before.
4) Just be confident and try to enjoy yourself. Students like to watch confident people at work and that they are in control and know what they are doing.
5)Have a banana apparently!!!
Good Luck. You'll be fine I'm sure.