You might not be signed in to iCloud. Go to Apple menu > System Preferences, then click iCloud. Click "Forgot Apple ID or password" and follow the onscreen steps. read more
To create an Apple ID, you will need to enter a valid email address. This will be used as a contact email, and will also become your Apple ID. When you log into a service that requires Apple ID, you will enter this email address and the password that you create. read more