How do you get a poster?

Question:hey, what's up? I'm really new to this advertisement thing.
So i found out that they do posters in office max. I was just thinking, since I'm going to use the file from my pc, would it have to be in a certain format? Does it have to be a certain type of file and does size matter? Should i make the width and length the same as the poster's?

Answers:
Try to google it, they have alot of info.

k