It's a job for a Mac tech, because your computer is new and still under warranty. If you open the iMac up to get at the audio socket with the broken mic plug, you'll void your warranty. Don't try messing inside that tiny aperture, and don't try to push the broken part through, it may get stuck in delicate electronic parts inside the iMac. It's bad luck for you - put it down to experience.
i grabbed some tinny tweezers and pulled it out .... i managed to get it out and its working perfectly now