How can I stop being so nervous at work?

Im 16 and this is my first job. I work at a grocery store as a bagger. Every time I go to work I get really nervous. I'm not exactly nervous about bagging, but I am nervous about not knowing what to do. After I punch in should I ask the supervisor where to go? cause that can be awkward. but most of the time there are no cash registers open for me to bag at. Therefore the manager has to tell one of the baggers already working to take a break. UGH. I don't know why I freak out so much about everything, but i am constantly nervous. Does anyone have any ideas on what I should do about work? or nerves in general? Thanks!