Removing vocals from a stereo track (or conversely removing instruments from a track, leaving the vocals) is difficult to impossible.
To do it properly really means having access to the original multi-track recording studio tapes or files. When performers have backing track tapes, they have had specialized mixes made from the original ‘stems’ in the multitrack.
You can do a few things with EQ and with center canceling — assuming the vocal is panned dead centre. However neither of these is terribly effective, and both will damage the sound of the instruments as well.
Equalizing the track to remove the frequencies where vocals mostly reside may remove vocals but it will leave a big hole in other instruments that also generate the same frequencies.
Center canceling is where you take one channel of the stereo pair and invert its phase. Then any waveforms that appear identically in the left and right channels (that is, anything panned to 12:00 dead center) will be cancelled out. Problem is, that will kill the bass and kick drum and any other instruments that are also in the center.
http://www.ethanwiner.com/novocals.html
You can try some editors like Audacity (free, Win and Mac): http://audacity.sourceforge.net/
Instructions: http://www.youtube.com/watch?v=PqXiKYG3J7M
http://audacityteam.org/wiki/index.php?title=Vocal_Removal
http://audacityteam.org/wiki/index.php?title=Tutorials
AnalogX (free, Win only): http://www.analogx.com/contents/download/audio/vremover.htm
DJ Twist&Burn ($39, Win only) http://www.acoustica.com/dj-twist-burn/
Or commercial programs like Adobe Audition, Bias Peak, and others.