How should I dress for my first job interview tomorrow?

I'm 16 , and I just got called about an interview for an ice cream place I applied and we scheduled it for tomorrow. I was thinking of wearing dress pants dress shirt and dress shoes but my sister says i'll look retarded overdressed, so i was thinking something a bit more casual. ALSO, my mom says i'll need a workers permit but I just got out of schoolfor the summer and I cant get one from my school. Where could Iget one? Will I not get hired if I dont have a permit? Thanks