Whoever interviews you will probably have plenty of experience with people who are at their first time interview. I know it's nerve wracking. Here's what I do:

Bring a small, classy purse. In it, have your wallet with your ID/Social security card (or whatever you need to be hired), a pen, a backup pen in case the first pen doesn't work, and a note pad just in case.

If you don't have a purse like this, just put these things in your pockets. I usually put my ID in one back pocket, a small notebook in the other, and a couple pens in my front pocket.

They'll ask you to talk about yourself. Talk about your personality - are you friendly? Social? Quiet? Reserved? Talk about how well you do/did in school, any extracurricular activities that you're proud of. If you don't have any, then just talk about yourself in a positive manner.

Talk about why you would be good for the position. If you're applying for cashier, tell them how friendly you are and how you'll always be smiling while checking items. If you're going to be a stocker, tell them how you like working with your hands.

Tell them what your plans are for school. Are you going to college? Are you planning to? What's your major?

They'll ask if you have any questions. Always find questions to ask. It shows them you're interested in learning more about the company and the position, not just blah-ing your way through it. I usually ask my interviewer how they got their start in the company, what it's like to work with the company, etc.

Just relax. You'll realize once you're there that it's not as impossible as it seems. Good luck!

EDIT: I wouldn't say you need a resume at all, it's not a job where you need a lot of experience. Until you have some under your belt, it'll be hard to create one.

There are plenty of online templates and builders to help you put one together. Just google "resume builder" or "resume template" to see how they're supposed to look.