I have a job where I have to use the registers, too. I have been working in retail for about a total of 2 years (at two different locations.)
I have made a few mistakes (once involving not swiping-or choosing the wrong tender of a card.) The customer apparently got away with 100 dollars worth of merchandise. Though, since I am a good, honest worker, they just had a talk with me about it. Mistakes happen.
You get better with more experience. Just try to take your time and not stress too much. I know it's hard because retail can get very busy! But even if it's extremely busy, it's better to keep your cool. If you can, call for backup cashiers or page a manager and say you need some help in your department. They left me alone on Valentines Eve at a cashier in a busy department. I called for help, no one came, so I called the manager and asked if she could get someone to help me.