I echo what everyone else has said (either, both), but it's rarer for me to start with a melody than to start with words or to compose both together, which usually happens when I'm noodling around with an instrument. Sometimes I will write some words and a melody will play in my head. In my best songs after I start them the verses just roll out in the screen of my mind and I don't have to do anything but listen and write them down -- sometimes they keep going after I think I am finished with the song. In that case, I always incorporate them and am never sorry.

Sharyn