How do you get a song and turn it into just instrumentals(the beat)of the song?

You need vocal remover software. There are several free downloads available. Here is just one of them:
http://www.sharewareconnection.com/title...
Also, know that these don't always work that great. Sometimes, you can still hear the vocal tracks faintly in the background.

An instrumental version of the song must comprise melody and harmony.......not just the beat.
I am afraid I do not understand your question. The melody is more important than the beat.