I start my new job tomorrow and I'm a bit scared?
First of all, remember that your new employer doesn't want you to mess things up and get sacked in a few weeks time. They want a capable member of staff who can operate the cash register without upsetting customers and messing up the change - so, for that reason, they're going to train you how to do it properly. Secondly, modern EPOS registers have been designed in such a way that it's actually easier to do things right. Almost everything is automated and all you need to be able to do it count the change you're giving to customers. If you don't have experience doing this and if your maths isn't too hot, ask the person who trains you to show you how to "count back."
I promise you, it's easy! In a week or so, you'll be so comfortable with it you'll wonder what you ever worried about. :-)