I am a teacher of English, and I have been teaching for almost 7 years. I still remember my first day. Fortunately, the first day was with students of first level in high school, but the next days I had to present myself to students of four level too.
My tips would be: First. be natural. Dont try to pretend to be someone you are not. Of course you will be nervous, but dont worry, it is part of the first time. Just relax, and tell your story, tell your life, tell why you are a teacher, your expectations, and be nice. But dont forget to be formal with them. Be natural, but dont let your students be joking so much.
Prepare yourself in advance. Be ready to what you are going to say or teach. Dont go without a plan. That will give you strenght and you will look in control of the situation.
Try to do something nice, you must enjoy that moment, and your students will enjoy it too.
I learn then I teach