How do I change the file path for iTunes?

[Advance The Man]

I recently got a 2TB MyBook external and the artist names are not in a file. i.e. MyBook\U2, etc.

I want to do this... MyBook\iTunes\U2

This will allow me to create files and not have them mixed up with the music. I've messed things up so many times, I would appreciate an answer on how you do it.