When I start my Mac Mini I must use a password. How can I disable this?

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You only have to enable automatic login. To do so, go to System Preferences -> Users & Groups, press the padlock and type your password. Then, choose "Login Options" and select your user next to "Automatic login".

Remember that you cannot disable it if you are using an Apple ID to log in your Mac.

•Mac OS X - Enable Automatic Login:

https://kb.wisc.edu/helpdesk/page.php?id=39258

See page for images and instructions.

With only one user, you could do this.

However that's another security measure.

•OS X Yosemite: Login Options pane of Users & Groups preferences

Good luck! 🙂