Music first.
Let the melody tell you what the song is about (how does it make you feel?,happy?sad?)
Then, ..hum your melody to the music (you can use lyrics, but don't get attached to them)
I always try to keep The Polices "Doo Doo Doo, De Da Da Da" in mind. These 'lyrics' mean nothing, but SERVE the song perfect!
Great songs are not written in one sitting, and usually don't come as easy as throwing some lyrics up that fit the chords of the song (I've found this a problem with many punk rock songwriters).
The 'way' you sing something is what will get the audiences attention, and you may find, as I did...that everything sort of 'falls into place' when you have your melody nailed. (ie. guits/drums/arrangement)
(another song I keep in mind is "in the jungle". Everyone knows the 'melody'...)
Hope this helps,
-LIMiT