Well, since I'm a complete Newb, I'm not sure my opinion counts so much, but since I've actually been writing songs for some length of time, I figured I'd lend my two cents...
I usually (almost unfailingly) start with the music first. Mostly I'll come up with a cool chord progression on the guitar first, or I'll think up a melody in my head, then add the chord progression to it. Then I'll come up with a rough mental sketch of where I want the song to go, such as the emotional impact or feel I want the song to have as a whole. Then I'll draw up a song structure. Then I'll actually write the song in musical notation. Only then will I add lyrics to the melody, trying to keep in mind the original feel and/or emotions I wanted to evoke.
After having numerous discussions about songwriting with various people, I've found my approach is rather different from most rock musicians'. Oh well. I only do what I know...