Formal shirt, tie, smart trousers, smart shoes.
This is the minimum for pretty much all interviews, regardless of the class of the workplace.
You're trying to show that you are professional.
Hope it goes well! Be calm, collected, and polite.
Also, be sure to be 10 minutes early. It shows promptness and professionalism. Don't be any earlier, however, as that suggests poor time-keeping.
If you have to wait in a waiting room with a secretary/member of staff in it, keep calm and be polite here. The interviewer will ask the staff about you afterwards. The interview starts the moment you enter the building and ends when you leave it.
You'll do fine though, I'm sure!