Sometimes, when starting a new job, we all have those emotional barriers. You just gotta take the challenge, and be confident in yourself. In order to learn something, we all make mistakes- it's part of the learning process. Being confident at a job comes through knowledge and experience, and you'll get there the longer you work. Besides, if you make mistakes at a job, just learn from them and try not to do the same thing again.
Don't be afraid of this, it happens to everyone. Usually, in every job, before you start working, they provide you with training. Since you say you've worked at a store, usually store jobs are pretty alike in what employees do, so just think of all the things you learned from that job and use them in your new one to be a good employee. Well, I hope this was helpful. Good luck to you in your new job, and be confident in yourself!