On your Computer...
Open iTunes...
If you're using iTunes 11 click on the box icon in the top-left corner to see the " Show Menu Bar "
and from the Menu Bar select Store...

From the Store menu, choose Authorize This Computer.
(In earlier versions of iTunes, access this option from the Advanced menu).
When prompted, enter your Apple ID and password, then click Authorize.
See Here for full details...
About authorization and deauthorization