You need to sign in to iTunes 12 on your computer or the iTunes app on your iOS 8.3 device with your Apple ID before you can buy items from the iTunes Store or download your previously purchased content. To sign in to your iTunes account on the desktop version of iTunes, access the program's Sign In option. On your iOS device, however, use the iTunes and App Store section of the Settings menu to sign in to the iTunes app.
The Desktop Version of iTunes
Open iTunes on your computer and click Sign In on the top menu bar to open the Sign In To The iTunes Store dialog box.
Enter your Apple ID credentials in the Apple ID and Password fields. Click the Sign In button to sign in to your iTunes account.
Confirm that you have signed in to your account; your name replaces Sign In on the menu bar. Click your name on the top menu bar to display the Account drop-down menu, which has options to manage your account.
ITunes on iOS
Tap the Settings icon on the Home screen of your iOS device to open the Settings screen.
Select iTunes and App Stores from the list of options to open the iTunes and App Stores Settings screen.
Enter your Apple ID and password credentials and then tap Sign In to log in to your iTunes account.
Confirm that you have linked your iTunes account to the iTunes app; your Apple ID is displayed at the top of the screen. To sign out of your account, tap your Apple ID and then select Sign Out.