1. Next, fire up the program up on your computer, it should look something like
this:
Open a song file by clicking on File, then Open… and then double-clicking on the
song you want to use:
Here we’ve selected one of the songs that comes with Windows 7, you should
get something that looks like this:
…where you can clearly see the left and right sides of one single stereo track.

2. Now, to get rid of the vocals, you need to split the stereo track into separate
tracks; to do that click on the little down arrow at the top right of the track
information pane:
…then choose Split Stereo Track, doing so should result in a new track showing
up:

3. A new track information pane has appeared below the one above it.
Next, click anywhere in the bottom pane to select it, or make it current:
Then select Effect from the menu at the top of the screen:

4. … and choose Invert.
Then, go back to the track info pane and click the little down arrow again, only
this time select mono:
Then, do the same thing on the track just above it, making it mono as well.
Now, click the Play button…

5. …to listen to the results of your efforts.
Depending on the song chosen and the way it’s been mixed, you will likely hear
either just the background music part of the song, or in some cases the vocals,
but greatly reduced in volume to where you can barely hear them. In either case,
it should be good enough so that you can very easily sing along to to the music
Karaoke style.
Audacity is a truly powerful sound editing tool and the more you use it, the more
things you’ll likely find it can do for you.