First of all I should mention that downloading music you do not own is illegal. Limewire is also riddled with spyware/viruses, so unless you're familiar with what file sizes you want, I don't recommend using it.
That having been said, go to limewire.com, install the software (there's a free version), click on search, click on 'music', type in the song name you want, double click on the file, voila it downloads.
Itunes actually integrates with limewire nicely, as a limewire folder will even show up in your itunes.
I um....heard that's how it's done...I of course would never download music I didnt pay for...I totally just heard about it from a friend of a friend...yeah, thats the ticket.