The music usually at least takes some form for me first. Sometimes it is just a few chords and then an entire poem is spawned from the number of syllables in each line. I just write a whole poem and then the song changes and the whole thing comes together.

But in general I have more music than lyrics, so writing the lyrics for the songs is a priority so I can catch up with myself. It is not bad to start out with lyrics first, I just find that it is easier to write music because when you are practicing a sudden inspiration might hit you right then and then you've got some music. Starting with lyrics is a great idea. Like I said, I have done it where as soon as I know what kind of vocal line will fit in a song, I have written lyrics for it.

But what Jimmy said is hitting the nail right on the head- usually write to a theme or an idea, because then you can base the chorus on that theme and it will naturally become a cohesive song. If you don't choose a theme then your ideas will be sort of unrelated, which is fine and dandy but you will usually find that that is not very satisfying as the song nears "completion" and you will want to find a message or stance on something to portray.

After all, this is an expression of your soul so get your ideas out there. What are you passionate about? then start from there. :)