If you want to convert a Youtube video into a MP3 audio file, here is the link:
The web site will come up fast and you just enter the URL of the youtube video in the edit box and enter on convert. After it converts, you can download and save it to your computer.
Here is a website where you can convert even larger files:
When the page comes up, just use it like the other one, but this will allow you to convert much larger videos.