I get that sometimes. Worst thing about irrational nerves is that realising you're shaking makes it worse :/
Best way to get rid of it is to just
act confident. I know it's impossible to just 'stop' the nervousness by clicking your fingers, but if you act confident (e.g. eye contact, smile, be down to earth with them, slow confident movements etc) then your nerves will follow. Oh, and the best way to end nervousness permenantly is to go to parties, be friendly and generally meet new people by going out. Next time you're in town/the supermarket/the queue for the cash machine, try to start talking to someone. Generally people are keen to speak to you, keep that in mind.
At your job, just casually point out you're a little nervous to the customers and joke about it being your first day! Worked fine for me