I work at JB at the moment as a part timer and I can say the most important thing is to be yourself. Most people who work here (or at least in shop I work in) are generally very chill people, most youngish, so generally the interviewer won't expect anything formal etc. You'll probably get asked about your life, school and what you've done as extra-curricular then be asked to elaborate on what that involved, what you learnt, how you contributed, the usual.
You'll then be hit with how you found out about JB and why you want to work here (you can be honest here, e.g. talk about gaining experience and a bit of money). Might also prep yourself for a question on dealing with difficult customers.
That's it really, oh and dress casually, you're not applying for a business job.