Where a White Tux, with a purple Shirt, un buttoned to show off your chest.
they gonna give you the job just to entertain themselves.
Also think if answers to questions like
"Tell us about yourself"
"Why do you want to work here" (i've said before that i need the money and its convenient to where i live, stated something like time keeping and punctuality are easier, but a risky answer).
"What would you bring into the job?"
"Give examples of when you have given good service"
(I hit it, and I hit it gooood won't really do).
"Give examples of when you were put in a difficult situation involving others"
(I got a part time job as a magicians helper and this box was really small......)
"What hours are you available to work"
(they are not after a manager, just shop assistant work, show willingness to work when ever but don't over commit yourself as they will hold you to what you say here).
"what are you planning to do in the next couple of years?"
(they know your not doing this as proffession, so need to know roughly how long they can expect to have you, less than 6 months aint worth there effort to train).
Above all be confident, if its a proper interview in an office, they will ask if you want a drink. Ask for water (or anything on offer cold), if this is your first interview, you'll get nervouse and dry mouthed, also having a sip of water after they ask a question, gives you a second or two to formulate your replys without looking like your stalling and trying to think.
Some Questions you might want to ask.
"how much does it pay" shows confidence and you need to know, most jobs pay less for under 18's and woudn't have been mentioned in the interview.
"what exactly will the job involve" (see if you like the sound of it)
towards the end of the interview
"is there any reason why i wouldn't fit the job?" (gives you a chance to defend yourself, and include things they were looking for and you've not yet mentioned)
"When should i expect to hear from you?"
and remember to shake hands, when you meet them and when your leaving.
For further advice see a thread from 2 or 3 weeks ago about hand shakes.