A lot of people use a lot fo different orders. Some orders tend to give different results. I know a lotta peopel go chord sequence -> rest of instruments -> words (but of course, I can never think of lyrics :S),

but you will tend to get more melodic and possibly memorable songs if you start melodically, thoguh you may find the chords that fit aroudn the first melodies you wrtie will be very easy. Muck about!