You can find the tech specs for your Mac Mini model at
http://support.apple.com/specs/#macmini. For most of them, under the "Audio" subheading, you will see a line that says, "Combined optical digital audio input/audio line in (minijack)."
"Audio line in" refers to the "line level" analog audio standard, perhaps the most widely supported audio standard in existence. The jacks on almost all analog audio devices operate at line level. About the only ones that don't are those intended for conventional microphones, guitar pickups, & phono cartridges, each of which generates a very weak signal that requires a specific type of pre-amplification to boost them to line level. Guitar amps have built-in preamps, but they are not designed for microphones & produce poor results with them.
Various mixers & other devices have preamps designed for use with microphones, many fitted with 3 pin "XLR" jacks for use with pro & semi-pro mics as well as with 1/4" jacks for lower grade consumer ones. Some include both line level analog outputs that can be attached to any line input with appropriate cables & built-in USB interfaces to attach to the USB inputs of computers.
M-Audio,
Behringer, & many other companies make mic preamps with both analog & USB outputs commonly used for recording to computers, priced from as little as about $30 on up into the thousands, depending on the number of inputs & outputs they have & the accuracy of their components. The
Griffin iMic is another such device, at one time popular with Mac users.