In order for you to be able to access your iTunes account's media such as music, videos, book and more, you need to authorize your computer.
First of all, make sure that you are signed in to iTunes on your computer. To Sign in, open iTunes > Account > Sign In... You'll need to enter your Apple ID and Password.
Once you are signed in and iTunes knows who you are, you have the power to authorize and de-authorize computer on you iTunes account. You can only have a certain number of authorized computers on any one account, but we will address that in a minute.
To authorize your current computer, navigate to 'Account' > 'Authorizations > 'Authorize this Computer...'. Will will need to provide your Apple ID and password again to confirm this task.
You can only have 5 authorized computers on an iTunes account at any one time. If you wish to de-authorize computers on your account, you can do so in one of two ways.
To de-authorize a single computer, open iTunes on the computer in question, then navigate through 'Account' > 'Authorizations > 'De-authorize this Computer...'.
If you instead wish to de-authorize all computers on your iTunes account, go to 'Account' > 'View my account' > Enter Apple ID and password. Then, under the 'Apple ID Summary' heading, click 'Deauthorize All'. All of your computers will then be deauthorized, so this is a great way to clear spaces if you forgot to deauthorize old machines that you no longer have access to.
Now you can authorize new computers if you wish.