How do you authorize and transfer songs on iTunes?

Authorize and Deauthorize Computer controls are accessed from the Store menu in iTunes.

To authorize a computer open the iTunes software and select Authorize Computer... from the Store menu at the top of the screen. (Next to the "file, edit, view, and control" tabs)

Register for an iTunes / Apple Account, Register Your Mac (if you are using one).

So lets recap,

If you are running a PC with Windows, you must register for an account.

If you are running a mac, you must also register for an account. (But you also do so when you register your mac)