apple I.D.
how to sign into your apple account?
iMac, iOS 10.3.1
Where do you want to log into the account ? If on your iMac you can log into the account on its iTunes (Account > Sign In menu option), for the Cloud via System Preferences > iCloud, for Messages via its Preferences, and for FaceTime via its Preferences. If on your iOS device you can log in via Settings > iTunes & App Store, Settings > FaceTime, Settings > Messages, Settings > iCloud.