I start with whatever pops into my head first. If I come up with some music that I like, I'll play around with some ideas to suit the rhythm and the overall tone; if I think of a good lyrical idea, I'll write it down, then put some music behind it to see what suits. Jolly's absolutely right - the rhythmic pattern of the words is extremely important. If you want a tight tune, either write music based on the rhythm of the lyrics, or vice-a-versa.