Chill and calm down gianthead. Wear something smart but not ott. Maybe a shirt and trousers, I don't think a tie is necessary. I can almost guarantee you it will be a one on one interview. Research McDonalds if you want you, but I think everyone will agree that we all know a lot about them. Be enthusiastic, smile keep eye contact and remember to shake hands. Questions they might ask are:
Why do you want to work for McDonalds?
What can you offer to McDonalds?
What would you do if an unhappy customer was complaining about this/that product?
How will you be able to adapt in a fast paced environment?
And most importantly do you have any questions to ask? Always think of something, because this shows you have an interest in working for the company. Relax a bit, it's ok to be nervous because everyone in this situation will be, as a little stress will make you perform that much better. And finally just don't panic.
P.S. Where is the location of this McDonalds, because where I live they don't seem to be recruiting.