For you, just buy a cheap one from radio shack made for computers and plug it into the microphone input on your computer. Download the free program audiograbber to record to a wav file. In audiograbber click file-line in and then record when you are ready. Double Click on the volume control icon in your taskbar in windows then go to options-properties then click on recording and check off everything and hit OK. Then select the microphone volume meter and adjust the levels.
However, if you want the recording to sound professional than you need to spend at least $150 on a compressor, mic preamp and decent microphone.