Hi, Hanna-
if the microphone has a miniature plug like a headset plug, you can plug it into the microphone jack of your computer. There's almost always a microphone jack in back, but many computers have a second jack on the front.
On Windows, click on the "Start" Button, and then select the Controp Pane. Click on the "sound" link, and then click on "sound." If there's an option that says "manage audio devices," click it and then select "recording devices." That page should allow you to set up and test your microphone.

With a working microphone, you can use Sound Recorder, Audacity (freeware download), or other sound programs to record from that microphone.

Hope that gives you the general idea.

-Joe Offer-