The iCloud ecosystem is complicated, and it’s hard to know how much of your data is safe in the “cloud” and synced across your devices. We’ll walk through the process of setting up and explain what each feature does.
How to Sign Into iCloud
When you first set up your Mac, you should have been prompted to sign in with an iCloud account. If you’ve already set that up, you’re good to go, but if you’re not signed in you’ll need to sign in from the iCloud settings.
Open the System Preferences app—you can click the Apple menu at the top of your screen and select “System Preferences”—and click the “iCloud” icon. You can also find the System Preferences app on your dock and in your Applications folder.
These are the iCloud settings. If you’re not signed in, you’ll see this sign-in screen.
If you already have an Apple ID from an iPhone or iPad, make sure to use the same one for your Mac, or else nothing will sync between your devices. If you need to make a new account though, you can click “Create Apple ID” at the bottom to sign up.
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Source: How-To Geek