how do i authorize my computer

My boys each have an iPod that are linked to the same apple id. One has locked themselves out and thier iPod is now disabled. I am trying to figure out how to reset it but I don't want to loose the things they have purchased. So I tired to sink everything from the ipod that is not diabled to the itunes acct in hopes to be able to put it all back on the disabled iPod once I reset it. It tells me that I need to authorize my computer. How do I do that? Thanks