I have new iMac. Need to authorize it to use iTunes. Help!

I used tech feature that says to click on "Authorize this computer" in the iTunes Store Menu. But I don't see that listed, and I can't find a menu anywhere that lists that, either. Can anyone please tell me how to find this feature?

Also, evidently there are five computers authorized for this account, and I need to clear that list, but it won't let me see a list of authorized computers so I can clean house.