G:
This is pretty easy. I will show you a link that is pretty clear. If you still have issues I can walk you through it with screenshots.
Ignore what Neil said.
Good luck:
Authorize your computer using iTunes
When you authorize your Mac or PC, you’re giving it permission to access your apps, audiobooks, books, music, movies, and other content. You need to authorize your computer before you can use it to sync or play content that you downloaded from the iTunes Store.
You can authorize up to 5 computers, which means you can play your content on 5 different computers. To authorize your computer, you need to have access to it. You can't authorize a computer using your iPhone, iPad, or iPod touch.
If you authorized more than 2 computers, you can see how many are authorized.
Open your account in iTunes and sign in with your Apple ID. (Or open iTunes, click Store > Sign In, and enter your Apple ID and password.) In the Apple ID Summary section, you’ll see the number of computers that you’ve authorized with your Apple ID. You can’t see the names of those computers.
You don’t need to authorize your iPhone, iPad, or iPod touch, but you do need to associate your iOS devices with your Apple ID. Your iOS devices don’t use your available authorizations.
- Open iTunes on the computer you want to authorize.
- Choose Store > Authorize This Computer from the menu bar.
You might need to show the menu bar to see this option in Windows. In earlier versions of iTunes, you can access this option from the Advanced menu.
- Enter your Apple ID and click Authorize.
If your Apple ID uses an expired MobileMe account, you can still use it to authorize your computer.
If iTunes asks you to authorize your computer again, it won't use a new authorization. iTunes uses the same authorization for the same computer.