The 2011 iMacs have both an audio-in jack and an audio-out/headphone jack (3.5mm stereo, each). Each performs only its single function. One is output only, the other input only.
If you want to use a mic plugged into the audio-in, also make sure that in System Preferences > Sound you have set your input source to Line-in, otherwise your iMac will use its built-in mic.
If you have a headset with a built-in mic (like the Apple earpods w/3.5mm plug) with only one plug (it's a combined phone/mic plug) - and you plug that into the audio-out (headphone) jack, you will only get audio you can hear, the mic won't work. That's because the single plug connects both the headphones and the mic at the same time ... but your iMac's audio out is only audio out. You have to use a separate mic that can be plugged directly into the audio-in jack. I believe I have seen plug adapters that take the combined phone/mic plug and convert it to separate phone & mic jacks. Parts Express or Monoprice may carry them.
Another issue is that the iMac's audio-in is line level, not mic level; some mics may work directly with it but the volume may be low. You may need a mixer/preamp to use a mic with the audio-in port on your Mac.
Is there a specific reason you can't or don't want to use your iMac's built-in mic?