depends. what kind of mic you running? 1/8 (does it plug into the 'mic' jack on your soundcard? or is it a Condensor/USB/firewire?
Also, what O/S you running on? if you're on a mac. you're on your own there. heh. But if you are running through Windows xp or relative version..
First Make sure Nothing's on mute. Some microphone's have a volume control right on it. Make sure the mic's powered on and volume is Up if it has any wheels/settings on the chord or mic itself. I know it seems pretty obvious.. but you'd be suprised how many times it's overlooked.
Go to My Computer, Control Panel, Sounds and Audio Devices. Then the Audio Tab. Make sure that the 'Sound Recording - Default Device' Refers to the soundcard / port where you have the mic actually plugged in.
Then check the Volume & Advanced button underneath. Make sure that the mic volume is up and hasn't been disabled / muted. (Sometimes Mute can be good though as it just mutes the playback so you can't hear it as you're recording it.)
Lastly you can try going back into Volume Control. ( Start, All Programs, Accessories, Entertainment, Volume Control).
Go to Options, Properties. then Adjust Volume for RECORDING. Click Ok. Now the volume control main window will change. Make sure that the MIC is selected as the recording device (there should be a check underneath it) and not Line in (unless you want to run your mic through that port), or it could also be set as the STEREO MIX.. which when you hit record all sound that passes through your soundcard. - great setting for recording game music/website sounds etc.
And make sure the settings are right in whatever recording program you are using. If you go through all this, you should be good to go.