The best advice that I got actually came from one of the patients, a former nurse herself... Anyway's, she told me, "Ask questions, ask lots of them, and never stop.". Now this doesn't mean be a brat and ask questions just for the sake of it. There will be things that you will see that you may not understand, think about them, and then ask your teacher, the nurses on the unit, and your fellow students. The most important thing is to think
about it before you ask. You'll get much more out of your clinical experience by understanding as much as you can about it.
As for calming your nerves, just take a couple deep breaths. Remember, your teacher would not allow you to be there if they thought you were a danger to yourself, or your patients. It's definetely an exciting time to be a nursing student, enjoy your time.