Just pretend like you are talking to your teacher/instructor and try your damnedest to impress them with your wit, insight and knowledge. That's what I do - it shrinks the room when you pretend to have a one on one conversation.
I gave a ONE HOUR and fifteen minute presentation on a book on Tuesday. The teacher e-mailed me and told me what a great job I did, etc. I think it's easier if you go first - before anyone else gives their speech/presentation so that the teacher has no one to compare you to
Also, remember to leave an opportunity at the end to ask if you can clarify anything or if anyone has any questions.