The first day is always the worst for everyone. Being a cashier myself at one point, I think you're putting too much pressure on yourself trying to learn everything on the first day. In fact, I think it was wrong for your manager/supervisor to leave you alone like that, I can't imagine how that must have felt. For me, it took more than a few weeks to adequately learn even the most basic steps of learning how to use those stupid machines, but after a while, everything sort of gets burned into your memory and you will be able to operate everything without even having to think.

Now, onto the customers. For me, this was the worst part of the job. Unfortunately, the world is full of assholes and there is nothing you can do about that. I know it's difficult, but try your best not to take anything negative they say or do personally because you simply don't deserve those kinds of people interfering in any way with your life. They will not be in your life for more than a few minutes and you may not ever see them again. So, if they give you any shit try your best to ignore them and forget about it after (easier said than done, I know). On the other hand, you will meet many friendly customers that will brighten your day. I found that there were a lot more friendly customers than asshole customers, but it is human nature to dwell on the negative and we often forget any positive interactions we encounter. Also, if customers give you shit for not knowing what to do, just tell them you're new at this and still learning. Most decent people will understand this. If they don't, screw them.

Anyway, I hope you take this new job one shift at a time knowing that each shift will be easier than the last. It WILL get better.