I was terrified on my first day. I'm not good with people and am shy and introverted, but I decided to go in the deep end and work as a waitress - it wasn't great but that was 8 years ago. I'm still shy and still not so good with people, but in time I've learned at least a thing or two... and the first day is 8 years away - don't even remember it, despite it being the most harrowing experiencing of my life the few days before it came and passed

In most places they don't expect you to get everything in the first day. Just listen and learn, as atomicc says. Mostly you learn from doing, which only comes with time. Stay as positive as you can and keep your chin up!