I needed to do that same thing recently. I downloaded the trial of Adobe Audition. I don't recall exactly the steps I took but I do remember finding a tutorial on You Tube about it. One thing you should know is that once the song has been completed it could be very difficult to impossible to completely remove the vocals but you may be able to mute it enough to where it is virtually inaudible. That's the success that I had with it
I think MP3 files have all the tracks mixed already. So you cannot readily do what you are trying to do. And you can try above posters suggestion the Audacity which would have been my only suggestion, without trying it myself.
The only way to completely remove vocals from a recording without damaging other parts is to have the original multitrack recording the recording studio had when it recorded the song. But they aren't going to let you have access to that (If it's your song, that's different).
Or you can buy a karaoke version of the song, if it is offered.
What those vocal-removing machines or programs do is phase-cancel anything panned to where the vocals in the song are. But this has some serious drawbacks:
1. Anything else panned to the same location is also removed. Since the vocals are usually panned to center, and the kick and snare drums are also usually panned to center, the vocal remover removes them too.
2. If there is any distortion in the recording, the vocal (and the snare and kick) will leak through, especially in the higher frequencies, as a buzzing or tinny sound.
3. If the reverb of the vocal is panned elsewhere, it won't be removed.
4. Parts that are panned near to the vocal in the original recording will be diminished, compared to the parts that are panned farther away from the vocal. So the instrument balance shifts, favoring the harmony instruments usually panned wide over the melody instruments usually panned close to the vocal.
5. The resulting recording is always mono.
6. The data compression techniques used to make small mp3 files may prevent such devices or software from working correctly.