clean mac for disposal

The full list of what to do is here > What to do before you sell, give away, or trade in your Mac - Apple Support

If your Mini uses a hard drive, then first enable Filevault and let the encryption process complete before you erase the hard drive so your personal data cannot be recovered by someone using a data recovery app. If you don't enable Filevault and let it finish encrypting, then you should use the secure erase option in Disk Utility to write zeroes to the drive (assuming Disk Utility even lets you do the secure erase). The Apple article @MrHoffman provided assumes the drive is either using Filevault or the drive is an Apple SSD. If you don't use one of these two options to erase the drive, then someone can boot from an external drive to use a data recovery app to possibly recover some personal data.