Ok, jokes aside. You'll most likely be introduced to the service counter where you're going to have to learn all of the different lunchmeats, cheese, and salads that you'll slicing, scooping up, and weighing for the customers to purchase. It's a bit daunting, as there are a lot, but don't expect yourself to absorb it all on the first day.
You're going to be really slow at first, and it will be frustrating, but in time you'll gain confidence and knowledge that will speed you up quite a bit. Speed and efficiency is key.
If you have restuarant experience, and are good with people, that's a big plus. It means you can hustle, and still be pleasant with folks... and also shrug off the more ... um... we'll say "challenging" customers.
Just try to make it fun, and bust ass.. You'll be fine.
Expect lots of slacking from other employees. In my deli there are two employees in particular who will do ANYTHING to get off the counter. If someone is making sandwiches or trays they'll go stand next to them and "help" them even though it's a one person job and usually what they're doing isn't helpful. Sometimes they don't even make a pretext, they stand there and chat. But it seems that the workers in general hate waiting on counter and will try to avoid it. At least at my store.
If you're anything like the new hires in my store, the service counter will be your friend for the next week or two. If you've never done retail, you will quickly learn that there are good customers, average customers, and customers who want you to give them wine from a rock.
Depending on your shifts (whether you float, primarily morning, midday, or nights) you'll gradually learn how to package products, make sandwiches, salads, and trays, take orders, how to close, and maybe even fry chicken.
Everything else everyone said is not a lie; you'll probably see at least half of it. At least.