Without getting into the illegal end of things, i will explain it in a PR acceptable way
You can get Limewire from download.com, download this and install it. When in limewire, choose search for audio. Enter a name of an artist or song, or even both, and click search. To download simply double click on the one of your choice.
Now you want to be sure you never download any mp3's you dont own a legal copy of. IE if you own the cd, it is legally acceptable to posess an mp3 of a song from said cd on your computer for backup purposes. I obviously dont own all those cd's, but were going for PR
Be sure to install antivirus programs before using any p2p, downloading, etc application, as some people upload nasty stuff disguised as music. Always look for music who's filenames are accurate and neat. For example, lets say you search for ACDC, Hells Bells. Some results may be:
acdc - hellz bells(1)
acDC - hells bells (REAL OMG)
etc etc. I dont download them unless they look like this:
ACDC - Hell's Bell's. The lack of commas would be ok, but a file named properly with capitals and such show the creator took his or her time, and you are much more likely to get a quality file.