1) Everyone's nervous the first time they use a cash register.
2) Everyone will have that moment of panic when they hit something wrong the first time.
3) You will definitely panic when you have a customer waiting, and you need to change the roll of tape (receipt paper) the very first time.
4) With practice, it'll become second nature.
5) People will get mad at you for taking so long, but there are others who'll be very patience and considerate when you inform them it's your first day on the job (or that you're still new).
6) Use one machine, you used them all. Most registers are very similar, with the only difference being the physical design, and whether or not a credit card machine is built into it. Most stores now have a card reader for you to swipe it yourself, but there are still some places with machines that require the cashier to swipe the card themselves.