As a 17 year old applying for a part time job I don't think you'll be expected to wear a suit, but make sure you turn up smartly dressed (I'd recommend a shirt and tie as well - it doesn't hurt, and the interviewer will be impressed that you made an effort) and most importantly,
TURN UP ON TIME!. Arrive 5-10 minutes (no more) before your interview, shake hands with your interviewer when you meet them and maintain eye contact.
Try and answer all the questions they ask honestly and without hesitating or going "ummm". They will almost certainly ask you why you want the job so think of a good response to that one - "My mum told me to get a job" isn't a valid reply

. They will also probably ask you if you've got any questions about the job, it's wise to think of a couple and to try and make them sound as if you're interested in furthering your career with the company, even if that's not really the case - companies want their staff to be loyal, because recruiting is a pain in the arse. For example "Do you think this could lead to a full time position?", "Will I be able to learn new skills that will be useful to me in future?". That type of stuff always goes down well, at least it does with me when I'm conducting interviews.
HTH and good luck!
Cheers,
Plug