I always get the first day jitters...I think it is natural and helps you over perform.
Or just sweat like a pig...don't forget your deoderant!!
I usually use the first day to introduce myself, get to know them, play games and find out what they what to learn, goals and history.
Once students know you and are comfortable they will be more receptive and learn more.
Don't worry, I always get nervous the first day.