being a cashier at a supermarket / grocery store. is it hard?

It can be stress full but it's not that hard to be a cashier. You should get some training before they throw you on a register. Usually they have scanning aids or code numbers listed some where to help you look up a fruit or vegetables if there's no code or number on it. Bar codes usually have any where from 5-10 numbers on it. If you don't know what it is you can always ask. But do get to know more of your fruits and vegetables. You should also know how to count money and give change. If you do encounter an upset customer try to understand and make them happy by following procedures, if you can't do that then call your manager / supervisor, let them know what happened.
I can name 3 types of tomatoes that look the similar (vine tomatoes, heirloom tomatoes and field tomatoes they are slightly different in size and colour) and I don't even work for a grocery store. I just like trying different foods. Over time you will get to know what types of produce they have.

Grocery stores are a better option for those without working experience. Cashiering is only slightly difficult when your first learning the ways but still it's probably the easiest job if you can get just relax and get over your nervousness. The hardest part I found was over time still delivering genuine customer service. Because people can get annoying real quick. You have to stay calm and kind to even the most ignorant people. Though your favorite customers will always make your day. Once you can transfer departments I would do it. I actually started out bagging groceries and even though that seems like a crappy job I enjoyed it more than cashiering because I didn't have communicate as much and I was able to do other things to kill time and get away from the floor. You have other options than grocery retail but grocery retail is the best place to start because you have chances to move up in the company.You could work in restaurants, I say avoid fast food if you can.

Harder than it looks, I only realised when supermarkets started doing those self service machines, though I'm sure you'd get used to it as most cashiers don't make any mistakes.

No, not at all. The only job that's easier is pizza delivery.