Record your voice or open an audio clip.
You can record your voice within Audacity by pressing the record button.
If you have your file already, open it with audacity.
If you want to use a mp3 file, keep in mind Audacity doesn't support mp3 natively -
you need to download the LAME encoder plugin.
ow choose the part you want to use (I guess you are familiar with play buttons).
When you decided for a part, scale (see the arrows) the wave so it's big.
Now take a screenshot of the wave
(mac user do it like this
, windows users like that
, linux users are nerdy enough to know how this works)