Fresh breath is so important...

Just be yourself, tell why you would be the one for the application, tell it is your favourite supermarket and you would like to contribute to it's success. Show that you are eager to get the job. They probably want to know more about you in general as well (hobby's ,school etc). Not much else, just don't be a smart ass.

Dress nice
No jewelry
No chewing gum
Make sure you smell good
Talk intelligent, not as if your talking to your friends
Sit up straight
Use best manners
Ask about advancement opportunities
Within 2-3 days of interview, call store to talk to interviewer about position. (Assuming you weren't offered anything yet)

well superchunk gives good advice

look smart,be polite and they always like that you can be part of a team so say that

but good luck,always best to be earning some money and starting young

and don't be late

Ah... the tricky one.. lot's of people fail with this one:

Basicly the answer is like this: I meet all the essential criteria for this job, "plus"

The "plus" is about:

Additional skills and experience above requirements
Additional knowledge
(Proven performance) doesn't count in your case
(Proven values of work achievements) doesn't count in your case

The point here is largely about being competitive with the other candidates. You don't know them so it's your qualities & skills that have to shine through with your answer. You have to show yourself as a superior candidate...

To add to this I'd say:

Well rested - You're not only going to look better but you'll be more alert and able to recall things much easier

Read up about your prospective employer and model your answers accordingly. Do not be afraid to ask questions - show interest. (I realise it may not be applicable in this particualr case but it's good for future reference)

Good luck!

interviewers do not like it if you query the BNP badge on the office floor, trust me, they really, really dont.

oh, and whatever you do, do not bring up the weather, I learnt that the hard way, urgh, I never, ever want to hear a bald middle aged man ranting about the rain for 10 minutes ever again, fucker didnt even give me the job >_>

more seriously: just try and be relaxed as you can, if you are stressed you make more mistakes