The Mac Mini is one of the most underrated home server platforms available. It's small, quiet, power-efficient, and any model from 2012 onward is powerful enough for the jobs most people actually need: file sharing, Time Machine backup target, media server, Pi-hole, or a lightweight VM host.
Before you start: wipe it and do a clean install. Even if it's your own machine, a fresh macOS install gives you a known-good base. If the machine is old enough that Apple has dropped it from current macOS support, install the latest supported version — or consider Asahi Linux if you want a more capable server OS.
For file sharing, go to System Settings → Sharing → File Sharing. Add shared folders, set permissions, and your Mac shows up in the Finder sidebar of every other Mac on the network. For Windows clients, enable SMB in the sharing options. It just works.
For Time Machine, add an external drive, enable it as a Time Machine destination in System Settings → Sharing, and every Mac on your network can back up to it over Wi-Fi. This replaces a $300 NAS for most households.
For media, install Plex or Jellyfin — both run natively on macOS. Point them at your media folder. Plex is more polished and has better apps; Jellyfin is fully open-source and free with no subscription. Either one turns the Mac Mini into a media server your TV, phone, and tablet can all stream from.
Give the Mac Mini a static IP in your router's DHCP reservation table and enable Remote Login (SSH) in System Settings → Sharing. Now you can manage it from any other computer on the network without ever plugging in a keyboard or monitor.