how to authorize this computer

Via the Store menu in iTunes. If you have iTunes 11, which for some unfathomable reason hides the menus by default, press Control-B and you'll then see the menus.

Regards.

I am there and deauthorized ALL because one I just sold was still on there; and I just want my new one authorized. I am in the store and there is no option to authorize; only deatuhorize. Thank you.