An a capella version of a song is one where the backing music has been removed, and only the vocal part remains. They're used primarily in the creation of remixes, and sometimes by DJs to blend them with other tracks, without the original instrumentation getting in the way. Creating an a capella from a complete song is a tricky task, and it is unlikely you will be able to get perfect results. What you can do is maximize the positive elements and minimize the negative as best you can.
Open your sound-editing package, and import a copy of the song you wish to convert to an a capella.
Click on the "Edit" or "Tools" menu and look for the "Pan/Expand" option. Run the process, which will separate the track into its left, right and center channels. Click the "Mute" button on the center channel, which will remove the majority of the vocal, causing it to be lost among the other instrumentation.
Export the instrumental to a new file on your computer by using the "Export" or "Bounce" option in the "File" menu, located in the top left corner of the screen.
Create a new project in your audio software and create two channels. Import the original track into the first channel, then the one with the muted center channel into the second.
Highlight the second version of the track and open the "Tools" or "Process" menu. There should be an option for "Invert Phase." Click this and allow the process to complete. Inverting the phase of the second track will cause it to cancel out the backing track on the first channel, leaving only the vocal behind.