In order for you to own the song 100%, your co-writer would have to sign a written agreement that assigns 100% of all of their right, title and interest in the copyright over to you (i.e., a transfer of copyright /copyright assignment). There would have to be consideration (exp. payment or some other benefit conveyed from you to your co-writer) in order for the assignment to be effective and that should be referenced in the written agreement. The consideration does not have to be a lot. It...