If your computer has a microphone input port
You're all set. Just plug a compatible microphone into the microphone input port on your computer.
What do we mean by compatible? Any microphone with a permanently attached cord that ends in a mini-plug should work. Note that many sound cards provide battery voltage for mono electret microphones on the ring of a stereo mini-plug. Check your computer manual to be sure if this feature is provided. If it is, be sure to use a microphone that is designed to accommodate this.
|A mono mini-plug
||A stereo mini-plug
A computer headset-mic combination is pretty much guaranteed to work. These microphones are not the highest quality, but can be had fairly inexpensively. If you're just getting started and want to experiment without spending a lot of money, one of these would be a good choice. Just don't be surprised if your recorded voice sounds a bit tinny or flat.
Desktop-standing microphones can be had for at little as $10 or as much as $500. Choosing a microphone is beyond the scope of this tutorial. Try searching the web for "computer microphones", visit your local computer store, talk to your friends, or ask questions on the Audacity Forum.
Unless you are prepared to spend extra money for a microphone pre-amp or standalone mixer do not buy a microphone with an XLR connector:
An XLR plug
If your computer does not have a microphone input port
Do not plug a microphone into the line input port on your computer. The volume will be way too low (the line input port does not apply the needed amplification to boost the very quiet signal from the microphone). You won't break anything, but you will be very frustrated with the results.
Option 1 - Buy a microphone to USB adapter
These devices plug into a USB port on your computer, and have a microphone input jack (usually 1/8").
Some options are the Griffin iMic and the Roland Duo capture UA-11 UA-1G.
You will still need a compatible microphone. Be sure to carefully read the specifications of any adapter you are considering and make sure you get a microphone that will work with that interface.
Tried it on a different computer
I even went so far as to install Reaper on a friend's computer with Windows Vista with Service Pack 2 (I have Windows XP with Service Pack 3) and on his computer in the Audio Device Settings, there is nothing in the Output Range. It says first 1: Not Connected and last 2: Not Connected 2. The built in microphone seems to work and as I record I see the bars moving up and down, and the sound increase on the horizontal track. But when played back there is no sound. I tried to switch it over to the Ensoniq, but there seems to be no provision for it other than Enable Inputs, and there is nothing there but: first 1: HD Audio Microphone 1 and last 2: HD Audio Microphone 2