right click on the song, click get info. that will tell you the file name and location. Using windows explorer, you can find that file, and from there you can e-mail it.
there's no need to poke around in the file system - simply select the song(s) in the itunes list and then drag from within itunes to the desktop. it may even be possible to drag directly to the email application.
alternately, from the file menu (or right-click), pick 'show song file' and it will open the appropriate folder. the keyboard shortcut is control or command + r (for reveal).
Like others have pointed out, you can only email purchased songs.
any song can be emailed. however, purchased songs won't play unless the recipient's machine is authorized.