Dress in a manner that is professionally appropriate to the position for which you are applying. In almost all cases, this means wearing a suit. It is rarely appropriate to “dress down” for an interview, regardless of company dress code policy. When in doubt, go conservative (is this starting to sound familiar?).
Your clothing should always be neat, clean, and pressed. If you don’t have an iron, either buy one or be prepared to visit the dry-cleaner’s often.