Grab Audacity from their website here. Audacity is a great free program that lets you edit audio files. Once it's downloaded and installed, run the program and open up the song you want to modify. Make sure the song is in stereo not mono, and that it's in the highest quality you can find.
The audio file will be joined so the first thing you need to do is select from the pictured drop-down list "split stereo track". This will give you a left and right audio track.
Now highlight the right track. Make sure you only select that one track. You can do this by dragging the cursor over the waveform. The whole track needs to be selected.
With the right track selected, from the top effect menu now select "inverse".
Now back to the drop-down list for the audio tracks. Click it again and choose "mono". Do this for both left and right audio tracks.
That's it, you're now done. You can play the audio from Audacity or export it from the file menu. What's happening is by inversing the audio of right audio track anything that has the same sound in the left audio track will be canceled out and become silent. Left and right audio tracks are mostly different, except for the lyrics which should be the same in both. This isn't 100% so results will vary. Try it on several different songs to check out the results for yourself.