A regular microphone will not work with your iMac by just plugging it in due to the "input" being a combo Apple Mic/Line-In (someone correct me if I'm wrong!)... An Apple Mic will extend further than a typical 1/8" microphone or line jack and makes a connection to a mic pre-amp that's built in... My only other suggestion is putting your microphone(s) and equipment into a pre-amp of some sort and then into your Mac (You won't get a high enough sound level into it otherwise)...
Once you have sound going in to the Mac, I would use Coaster (search
http://www.versiontracker.com for it) to record the audio.. It will record the audio coming into the Line-In as one big .AIFF file. From there you can use a variety of editing programs (ProTools, Peak, etc) to mix the sound up... From there, once you are producing .AIFF files, you can burn a cd out of them by using a program such as Roxio Jam (my preference or even Roxio Toast)....
I know this is a broad overview, but the key is that a mic going into your mac won't work that great and that you should use a mixer of some sort to blend the signal and dump it in (unless you're going to be doing multi-track recording)...
-Mike