If you've got a copy of a song and you want to remove the vocals from the song for some reason... maybe you want to sing over it yourself or maybe do something else with it, you can remove it pretty easily with Audacity, a free program that you can use to edit audio files. Granted, this method usually doesn't work that well and when it does the sound quality really suffers, so before you do this, I suggest searching the web to see if there's a good quality one out there, but if you can't find one and you really need to do this for whatever reason.... Well, you can use this little handy guide.
First thing you need to do is
download Audacity if you haven't and open it up, and then open up whatever song you want to remove the vocals from. I better mention this now, this method only works if your copy of the song is in stereo, sorry but there's pretty much no way to remove vocals from a mono version of a song easily.

Now we have a stereo track of a song, pretty simple.

Next, we need to split the stereo track, as seen in the picture. Just click the name of the track and select split.

We have two tracks now and we need to invert one of the tracks, so just click on one to select it (make sure you have one of the two selected, no more, no less) and then under the "Effects" tab, click "Invert". It might take a few seconds but then the track will be inverted. I selected the Right (bottom) track out of habit but I'm pretty sure it works if you invert the Left track.

Next make both of the tracks into Mono, click on the tracks name and from the drop-down menu select Mono. Do this for both tracks, and then you'll be solid

And you're done! Just a quick listen to the track....

And there are still remnants of the vocals there in mine, and probably in yours too. And the quality is dropped a bit too. But it will still work pretty well if you plan on singing over the vocal remnants or mixing it along with something. It's just probably not that great for simple listening. A nice general rule for this is if it's sort of a high-end song that probably used a bit of editing, it probably will be far from perfect, but if you downloaded it from someone's myspace, well then hey, it might work almost perfectly as intended. Just make sure you export it as a wav or mp3 or whatever if you really like it.