Try to first tackle the superficial level, that is, dress appropriately and seem confident. Don't seem incredibly arrogant or self-doubtful. That's the easier part.

The harder part is what to say. Generally you can use one to two minutes to "sell" yourself, that is, list the qualities and summarize past jobs or volunteer work that make you qualified to do the job. See what the position is and think what qualities are needed. The employer would want to get the idea that you're reliable, hard-working, friendly, punctual and efficient worker who can help out other workers or customers even when you're done your work. The idea isn't to go zooming around like a roadrunner and directing the customer to a certain part of the store, you want to be kind and curtious.

Since you'll be dealing with customers of all varieties, you'll probably be asked to role-play out how you'd handle certain situations. It's more than simply smiling and being friendly, you need to be insightful and have enough knowledge (which you'll eventually gain).

Lastly, and if you're good at this then I'd advise you to do so, read the interviewer and at the same time, understand what you're doing. The interviewer is going to note your body language, tone, etc..., and you should too while knowing what you're doing and molding yourself to what hopefully will please them. Since you probably don't know the interviewer at all, you'll need to get some talking between you two to quickly try and figure them out. But, if you're not good at this, then I wouldn't do that much of it, however, do be aware that the interviewer will be noting your speech and behavior.