The problem

The usual microphones used in PC's and headset-mic combos won't work with the mac mini--the mac mini mic input requires line level input, meaning the mic needs to be powered. The usual mic input for PCs are mic level input (about 1 millivolt, the mic doesn't need to be powered). Bottom line is, if you have an existing mic which has worked with your PC, chances are it won't work on the mac mini.

Possible solutions

There are lots of solutions for this problem, just google the term "mac mini mic" and I'm sure you won't run out of options--there are USB mics, and for sure you will run into the iMic from Griffin; but this guide is not about the iMic, I wasn't inclined to buy a condenser mic or the iMic simply because I use the mic on the mini just on ocassions.

The hack

1. Find an old voice recorder. There could be one (or two) gathering dust in that sealed box of yesteryear-gadgets. I found an old Sony TCM-150. Any recorder with a built-in microphone should do it.
2. A 3.5mm stereo jack (mini jack) with both ends as male connectors. This can be easily procured at any audio supplies store or hardware supply store. I got mine from ACE hardware.
3. Once you have procured the 2 items above. Connect the Mac Mini and the voice recorder using the stereo mini jack. You might want to check the batteries on that old voice recorder, I hope you did not forget to remove the batteries when you locked it away for keeping. If the dry cell batteries have leaked, the voice recorded may not be usable anymore.
4. Power the voice recorder. Put in record mode (you can press the pause button so that it actually doesn't record, we just need the mic)
5. Test it. I tested mine by using it in a skype call. In fact, that was the only reason I needed a microphone for the Mac Mini