He is. <_<
Wear a suit - or at the very least, dark trousers and a shirt. Don't wear jeans. Don't wear a tshirt. There's no such thing as being overdressed for an interview, even if it's for a casual store. Make sure your shoes are clean. They will, whether unconsiously or consiously, be looking at your appearance to see if you're neat and tidy. Shave if you need to.
Research what sort of stuff the store sells and what your role might entail.
Have some answers for typical interview questions ready ie why should we hire you? Tell me about a time you've worked in a team (even in a sports team at school etc). Why did you apply to work here?
Get a good nights sleep and be awake and ready.
Get there 15 minutes before it's due to start, not very early and most certainly not late.
Smile and be genuine with them. Remember they're only people - they'll expect you to be a little nervous but try your best not to be a stammering wreck.
They'll appreciate you're young and don't have experience, try not to worry but have some examples of teamwork situations prepared, mainly from school in your case.
Ask questions if they ask if you have any. Even if it seems as though they've covered everything, ask. Good questions might be about the training system they use, ask about salary if they've not mentioned it (this isn't as rude as it sounds like it could be) and about potentially picking up extra hours if you're available. Write these questions down if you think you'll forget.
Shake their hand when you enter and when you leave.
If they ask if you'd like a drink of water or a cup of coffee, say yes even if you don't. It will give you the opportunity to take as sip if they ask a difficult question where you need a second to think of an example/answer.
Take a copy of your resume with you, even if they already have one.
Most importantly, relax. They're looking for someone to fill the spot as much as you're looking for a job. They wouldn't be interviewing you if they were looking for someone with tonnes of experience - they know how old you are.

Good luck and let us know how it goes.