How can I change my primary account on iTunes from one computer to another computer?

I opened an iTunes account years ago when I had a Dell, and after the Dell crashed, I got a new Macbook and authorized the mac to play all my iTunes music and whatnot that I had bought on the Dell. I don't want this computer to just be "authorized", however. I would like to know how to set this mac as the primary computer so that I can then authorize 5 additional computers to play my music. Does anyone know how to do this or have a phone number for Apple to get in touch with someone who can do it for me? Thanks very much!