I agree to PostPone, removing vocals is one of the probably most tricky and complicated things as the human voice is very complex.
If you want to add your vocals to instrumental tracks, there are a few ways: For many songs especially in the rnb/hiphop-genre often they artists release the regular song and an instrumental, in times when we bought maxi-cds often next to the original song you got the instrumental on top.
On very, very old pop, blues, funk- and rock- and soul-songs you could try to work with the stereo-field as vocals usually sit in the 'middle' and instruments don't. That's not a rule but many acapellas have been created by fading out instruments on the left and right side of the stereo-field, some audio-editors offer even tools for processing exactly songs that way.
Using EQs might help a bit but usually you don't get rid of the whole vocal just by using EQs, that's my experience. Still depends on the track, sometimes you can have success, sometimes you do fail.
Some 'Wave'-Plugins for DAW offer the possibility to remove unwanted noise, like crackles, as these plugins can 'learn' it's also a way of how to remove vocals, usually you then need a fragment of the song with vocals only so the plugin can learn its frequency and then later filters that frequency out. The result can be okay or horrible, the more frequencies are filtered out, the more synthetic a track then can sound like a bad mp3-encoding or an old realaudio-media-file.
As todays CPU is so powerfull using spectral analysis could help as well, PostPone is absolutely right: But don't think with a few clicks it's done, usually when using special analysis you need to go thru the whole song second per second, find, select the different frequencies and blend/fade 'em out as best as you can so the instruments still remain. Some pro's use these methods for audio restaurations and some extract acapellas out of songs, but if it should be near-to-perfect you can spend countless hours just to work on a few seconds of an audio-clip to achieve good results. But it still depends on the source, for some songs it works perfectly, for some it doesn't.
So, as you see, there are different approaches and tools but none usually works like magic and does the trick by clicking one or two buttons.
AUDACITY (the free wave-editor) has - for example - a tool included to remove vocals/isolate vocals, you could give it a try and there are a few tutorials out there of how it works best.
VOCALZAP is an ios-app afaik, they claim they can remove vocals, haven't tried it for myself.
I slightly remember that even quikquak offered a plugin to isolate vocals.
So, good luck, when you're searching for that topic at other sites/google you'll find many tutorials and more tools which are worth a try, but be sure, it's not that easy. I remember someone mentioned at another forum that removing vocals out of a song is like removing the milk out of your coffee