How do I operate a register?

I recently got my first job at Goodwill. I had my first day which was mostly me doing a lot of lifting. My second day is tomorrow and there’s a chance I’ll be operating the register. I have a hint of social anxiety and I’m afraid to handle people’s money, as I don’t want to cost them anything more then they’re meant to pay. Are there any tips you can drop on me to help out? I’d really appreciate it.

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