How difficult is cashiering?

How hard is it? I mean it doesn't seem like rocket science, but what would it be like for an introvert who has never had a job before? I'm really scared of messing up in having to give change back; or do a majority of shoppers bring in credit cards? Do you get any say when it comes to your work hours? Can you ask if you can work in the morning rather than starting later on in the day or do you have no control over that? I'm already psyching myself out, but I really want to get some kind of job this summer. All information and more would be great!