Okay.. I shall share my secret.
Shh though
Basically, I go on youtube, search the song, copy the link.
Go on www.mediaconverter.org and press enter a link and paste the youtube link.
Then click next step.
Click mp3 on the arrow thingy.
Then wait for it to download, click save.
Then enjoy then song
You get 5 conversions a day though.
That part sucks.
i like the first persons idea, never hear how to do that, but if you want something a little simpler or if that doesn't do well enough, try skreemr.com