Cashier or stock clerk?
If it's anything like the grocery store I work at, it won't be bad at all. The people you work with will probably be around your age and you'll get to laugh and joke around a lot.
I'm a cashier and the #1 thing to remember is not to be a robot. Every customer, ask them how they're doing and when they give you the classic "good how are you?" (in a monotone voice) come back with "I'm awesome!" then something sarcastic like "As always here at shoprite!"
Second most important thing cashier-wise is just take your time, especially when first starting out. You aren't going to know any of the codes for the produce, and that will take awhile to memorize. When you get that order that has like 1000 vegetables, don't be afraid to tell the customer "Sorry I'm new, can you tell me what this is?" If you don't know what kind of fruit or vegetable it is. A lot of times there will be a sticker on stuff, but you need to know if it's by quantity or by weight. Things like watermelons, cantaloupes, lemons, and limes are by quantity. Things that vary in size more like apples, green/orange/yellow peppers, lettuce, ect are by weight. You'll learn but produce is probably one of the things that will be annoying first starting out.
But on the subject of always taking your time, counting change back and making sure your drawer is right is like #1. Stores don't like to get fucked around with money wise. They know you're new so there will be a more lenient time when first starting out. But impress them, double count all the change back and just be sure your drawer is right.
They don't just throw you out there either, they usually have an orientation day, a day of training, then a day where you're actually doing cashier work, but with someone right next to you to answer any questions you might have.
I also work as a stock clerk sometimes too, which has less face-to-face people action and generally more physical labor. Depending on how big the store is, you might have to be out getting carts one minute, then trying to put some stock on the shelf the next minute, to mopping up the mess left by a jar of pickles someone dropped on the floor the next minute. My advice for stock, if you're doing that, is to just be ready for anything haha. Don't cut corners, if they want you facing the shelves 2 high, 2 back, even though it SUCKS for glass spaghetti sauce jars and glass applesauce containers, just do it. You make yourself look good and they know they can trust you.
Ughhh why did I type so much? I feel like I didn't really say all that much though, so if you want me to tell you more about what I know as far as grocery store, I think I could probably help. Shoprite isn't the same as my store obviously, but a lot of similarities exist I'm guessing haha.