Just like the first day on any job, one is going to be a bit nervous. As an instructor's we have to keep in mind that this is a new experience for the student's and they are as just as nervous. I think a really good way to break that barrier is to come to class early and stand outside the door and welcome and greet the students before they even enter the class room. Also while giving everyone a chance to get settle in, start a conversation about the weather or some hot topic that made the news. This always seem to work and the students don't seem so nervous about being in a new place for the time.