A smile breaks the tension. Be in the classroom, dressed appropriately. Be organized. Talk with students one on one. It is not about you, it is about the students. Intorduce yourself and explain your background. Reinforce the fact that you are there to help them suceed. Reinforce that you are therre to learn with them and from them as they from you. Next, insure everyone is in the correct class. Explain teh objectives and how you plan to help them absorb the material. I frequently explain my teaching practices and this relaxes the students knowing they will have fun learning. Thirty years of experience is hard to roll into one expanation. Learn from yourself and improve each and every class.