Some older songs mixed in stereo had instruments panned to either the right or the left (usually right) and vocals panned to the other side. The first trick should be to mute one side of the music to see if either the instrumentals or the vocals have been isolated. If the vocals have been isolated, switch to the other channel, delete the channel with the vocals, pan the instrumentals center and export the song. If this worked, congrats!
In all probability, if the song was recorded after about 1975, this trick did not work (however, it's easy enough that it always deserves a shot). Continue following the steps to remove the vocals.
Since the track should already be open, make sure that there is access to both the right and left track entirely individually. For example, in Audacity, it opens stereo files as one track and the user must split the stereo tracks in the menu. Specific audio work stations vary, but the first step is to have a left channel and a right channel.
Select only right channel (most likely where the vocals are) and apply an inverter effect to it. The inverter effect causes it to be panned completely opposite of where it is. This should create two identical channels of instrumentals only.
Either merge or export the track and the vocals should be gone or next to inaudible. Voila, free karaoke track!