We will assume your mic is plugged into your sound card for now, and that you have Windows XP.
Go to Control Panel >Sounds and Audio Devices >Voice tab. Make sure your Default Device is your sound card. Then click Volume. Recording Properties should show up. Click Options, and check the box to show Mic In, then click OK. Find your Mic level slider, move it to halfway, and make sure the box is checked to select it as your input. Close all those windows.
Now launch Audacity. Up top, you'll see a dropdown box. Pick Microphone. You'll see a symbol for a mike right next to the dropdown box. Move the slider to the middle to start. Press the Record button in Audacity, the round pink dot. Talk into the mike and you should see the level meters bouncing. If they're too high, slide the mike level control to the left.
When you're done, hit Stop, the yellow square. Congrats! You've recorded your voice. Now you can edit the file and save it when you're done.
Tom