iTunes isn't just music software. iTunes is the software used to sync your Apple devices and download apps to your devices. You can only purchase media from the iTunes Store if you are signed in with a registered Apple ID. Once you log into your Apple iTunes account, you can also view your account history, billing information, recent purchases and other data. Access your account directly from the iTunes program window.
Launch iTunes. Click the "iTunes Store" option under the Store section of the iTunes sidebar. The iTunes Store loads to the main window.
Click the "Sign In" button in the top right corner of the iTunes Store window.
Enter your Apple ID into the Apple ID field. Type your password into the Password field.
Click the "Sign In" button in the dialogue box to log into your Apple iTunes account.