This is great for making your own Karaoke song
Now before i start i want you to know this won't completely remove the singer, but it will do a pretty good job of it so its worth a try.
Step 1: Download Audacity
First you need to go to google.com and search Audacity.
Then click on the first result which brings you to there homepage and has a download like right there.
I use the beta version but it makes no difference for this.
Step 2: Open a Song
For this process you need to make sure it is stereo or has two blue bars or else this won't work.
Step 3: Split the Stereo Track
Step 4: Invert Right Audio
To do this you need to click somewhere on the gray part of this bar,
then go to the Effects menu and select invert. (be sure the song is not playing when you try to do this)
The right track should look only a little different.