I bought a second hand mac mini late 2014 with 256GB SSD storage and with OS X El Capitan version 10.11.6. I do not know the seller and I assume the device is virus-ridden! I am mainly concerned that the previous owner might have installed spyware or just their mac mini was infected with one. This is the only macos computer I have access to. I have also access to a Windows 10 computer.
How can I wipe out the entire disk so that even if there are any spyware on the mac mini I can get rid of them?