Open System Preferences and click Users & Groups. Click Login Options, and use the drop-down option labeled "Automatic login" to choose your desired setting.

1) Open System Preferences.

2) Click Users & Groups.

3) In the bottom left-hand corner, click the lock icon to unlock the preferences pane.

4) Type a username and password with administrative privileges on your Mac. Then, click Unlock.

5) Click Login Options and choose an option from the "Automatic login" drop-down list.

NOTE: If encryption is enabled on your Mac, the option may be grayed out and cannot be enabled. This is expected behavior.

Outcome

Your automatic sign-in settings are set.