Download Audacity. You can find the latest version of the software at http://audacity.sourceforge.net/.
Open Audacity and import your song file. Just click on "File," then "Open" and then navigate to your song file. You can also just drag and drop your MP3 into Audacity. It'll take a few seconds for your song file to import. Your song will be displayed across a sound wave once it's imported.
Click on any part of the song and press play to make sure it is the song you want.
Click the drop-down menu to the left of your sound file. There will be a grey column to the left of the song file that has the song's title. Click on the arrow to drop the menu down and then click on "Split Stereo Track." This will split the track into two identical tracks.
Select the second track (bottom track) by clicking anywhere on the grey column to the left of the track. You can also highlight the entire track manually by clicking and dragging the cursor over the entire track. You should see a grey shade covering the track when it's selected.
Click on "Effect" and then click on "Invert." You will have to wait a few seconds while the track is inverting.
Click on the drop-down menu to the left of the track again. This time select "Mono." Do this for both tracks (top and bottom). Now whenever you play the track the vocals won't be completely gone, but they will be barely audible.
Save the project file so both tracks will be combined into one.
Open the track in Audacity again. This time you should only have one track with barely audible vocals.
Select the entire track and click on "Effect." In "Effect," click on "Amplify." An amplification box will open.
Move the amplification cursor into the negative. This will completely remove any soft noises on your track. Moving the cursor to negative four or five is adequate enough to remove the vocals. Your vocals will be completely removed.