Plug the microphone.

Keep in mind some desktop computers have sound jack holes on the bought, front and back, but operable only one of them.

Look into pictures below. It should be clear for understanding. Usually sound cards have fine colored and marked plug-holes. So you have plug read plug into read hole, and green plug into green hole.

If you sound card have strange coloring like black orange and blue refer to sound card documentation. Or just try every hole one by one.