When going in for a job interview always dress up.
Dress pants, a button down, long sleeved (even in the summer) shirt, and possibly a tie are the bare minimum. A jacket would not go amiss in an ideal situation, however.
If you don't have a pair of dress pants substitute a pair of dark jeans, however make sure they don't have flashy designs, of fade patterns on them. I would strongly suggest investing in a pair of dress pants.
As far as shirts are concerned, make sure it's ironed, button it up all the way, and leave the sleeves down. Make sure it's not a "clubbing" type of shirt, by the way. Standing out is fine, but you also want to be taken seriously.
A tie is kind of optional in some ways. I always prefer to wear one (a simple dark tie with a light, solid color shirt) because I want to make a killer first impression, but some people don't own too many ties, or not an appropriate one, and that's fine. You can probably leave the collar button popped open if you don't have one.
The jacket is optional,as I said, however I think it really enhances your professional appearance to wear one. It doesn't have to be matched as a suit, it can be simply a "sport's jacket", as long as it matches the rest of your clothes.
Dark (black or brown depending on your clothes) dress shoes are advised, unless the weather doesn't permit it (such as it being the dead of winter). Even then you may want to bring them with you and put them on in the washroom.
If you get the job and show up for your first day it's not a bad idea to be just as dressed up as for the interview, as you'll probably get the tour of the company and meet a lot of managers, supervisors, etc. At that time you can also see what everyone else is wearing and adjust your dress style from there on out.
Adapted for Women
It's been pointed out that I don't cover specifically female fashion. My advice is very generic:
- You can substitute a business-style skirt instead of dress pants (weather allowing)
- You can wear a blouse instead of a shirt, of course
- Don't wear very tall heels, thigh-high boots, or platform shoes, as those are not associated with professional environments
- A tie may or may not "work" with your personal style. I haven't met too many women who enjoy wearing them, and, lucky for you, you're not really expected to, so do as you please