Generally I get a tune... usually one line only..... in my head and fit some words to it. Then the four lines of tune seem to flow out and the words are forthwith written... HOWEVER. On given a lot of information on someones life and being asked to write a song about it... I wrote the words and hatched a tune later........ I don't think there is a definitive way to write songs, although I believe I did find it easier to write the words and add the tune.
Best wishes, Mike.