An Apple ID makes it possible for you to download content from iTunes and the App Store. It also allows you to sync reminders, calendars, and contacts through iCloud, and activates Facetime and iMessage across multiple devices. It’s not mandatory to sign up for an Apple ID if you want to use an Apple device, but it is essential to getting the most out of your iPhone and iPad.
Here are the steps on how to sign in to iCloud with an existing Apple ID on your iPhone or iPad running iOS 10.3 or later:
NOTE: The steps explained in this article can also be used for iPhone or iPad running on iOS 10.2 or earlier. The only additional step is to go to iCloud right after you enter Settings.
STEP 1: Launch the Settings app.
STEP 2: Tap Sign in to your iPhone at the top of the screen.
STEP 3: Enter the email address and password associated with your Apple ID.
STEP 4: Tap Sign In at the upper-right corner of the screen.
STEP 5: Enter your iPhone passcode if you have one set up.
STEP 6: Make sure your iCloud Photos are set the way you want them.
STEP 7: Toggle apps that use iCloud on or off, depending on your preferences.
NOTE: In iOS 11, you can use the Keychain password feature in the keyboard suggestion bar to add your Apple ID and password.