It is a scary first step to put yourself out there for the first time, after being home for so long. Take it one day at a time. Start by researching a lot before you go in. Watch YouTube videos on how to be a cashier, to prepare yourself.
It's important to remember that everyone makes mistakes, so just do your best and don't be hard on yourself. You will get the hang of it the more you work. The first few days will be the hardest but you will feel more confident and less scared as you keep going. You can do this.
I work as a waiter, and I am no good at socializing at all, don't even have any friends I can call close, just a lot of acquaintances. I just go there and do my job, most of the time I just act like an NPC lol. Of course, I've made a lot of mistakes starting out, and I still do most of the time. Just remember that mistakes happen and you'll get used to it. Also if there's a nice co-worker willing to help you that would be great. Also if you make a mistake, just say sorry and tell them it's your first day. Most likely than not, they will be understanding unless they are one of those rare losers that are in society just to cause trouble for others. I remember having to take an order from a customer on my first day, it's a good thing they were really nice and actually told me how to do it. Don't worry about it, as long as you don't somehow break everything, piss off all your customers or something, and be super rude you'll be fine. If you're really that worried, just mind your own business and do your job. If you get asked questions you don't know, just be honest and tell them that you don't know and that you're new and guide them to someone with more experience or something.
I know how hard it can be to do these things with anxiety, I've been there many many times before. You just have to go for it and say what you have to say and do what you have to do. I wish you the best and just try your best alright? Tell me how it goes after your first day if you don't mind. If you ever need to talk I always have an ear to lend.