If you don't want to use the built-in mic for some reason, note that iMacs that have an audio input jack (the new ones don't) require line level signals. Since most mics output much weaker signals than that, that means you will need some sort of preamp to boost the mic's output to line level to get it to work.
You can buy a regular (analog) preamp or a mixer to do that, or use something like the Griffin iMic, a USB powered preamp & analog to digital converter. There are many other options for USB powered audio interfaces, but that's the cheapest one.