For me, songs come from the air when you least expect them.
Inspiration is a funny, fleeting thing.
If you don't follow the whims of the muse immediately, the ideas can escape you. Usually, the music will come first and the lyrics will follow. Most of the time, the meaning of the song, lyrically, is like a puzzle that needs to be solved. I'll usually just start singing over the music and come up with a few key words and lots of gibberish. The idea for what the song will be about will usually present itself before too long.