As a small publisher, I can add to the original post that the song has to be KILLER before it should even be demoed. Get a critique from someone you trust (join NSAI for instance), play it for people who don't know you or don't know you wrote it and listen to their reaction. Have a great hook. Be original with rhymes. True/blue/you don't cut it.

Most of all, write something everyday.

Good luck, all!