What does a 2012 Mac Mini need for microphone input?
The audio-in port on Mac Minis is a line-level port, meaning that a standard PC microphone may not be audible without some sort of amplifier.

On any Mini from 2010 and later, you can use the Apple headset (the kind that comes with the iPhone), but you have to plug it into the headphone port, not the audio-in port. There are also adapters available that allow you to use any standard PC headset with that port.

On older Minis, you'll have to use a USB audio adapter. The iMic is a popular one, but there are many others out there.