General microphone troubleshooting
Not connected properly
Verify that the microphone is correctly connected to the computer. The microphone must usually be connected to the back of the computer into the microphone jack. As can be seen in the picture the microphone jack is usually a pink connector and has a picture of a microphone next to the jack.
Issues with microphone
If the microphone is correctly connected to the back of the computer, ensure that you have a working microphone. Some older microphones may require a battery; if you have such a microphone, verify that it has a working battery.
If you do not have such a microphone or the battery is a known good battery, verify that the microphone works on another device such as a stereo.
Sound drivers not setup properly
If the microphone is connected correctly, and nothing is physically wrong with the microphone, verify that the sound card is properly set up in your operating system.