microphone

what type of mic do i need for a iMac desk top pc yosemite 10.10 2014 model

Probably a USB mike. https://www.google.com/?gws_rd=ssl#q=usb+computer+mics

Since the iMac has a microphone built in, it would probably be helpful if you'd tell us a little about what it is you're actually trying to accomplish.

I do podcasting, and thus I felt the need to get a better-quality mic than the built-in one (though the latter is perfectly fine for Skype/FaceTime calling, relayed speakerphone calls and so on). I started with a Blue Snowball (which happen to be on sale for super cheap at Newegg.com at the moment), and later graduated to a Blue Yeti. I like them both very much.

I may have misunderstood what you are saying don't expect to see microphones in the input section all the time. Microphone names only appear there when they are connected. As soon as the microphone is disconnected it disappears from the list.