I am totally untaught, am not even sure of the notes of the strings on my guitar and play mainly to accompany myself singing. So melody comes first, then I hope to hell I can find the chords ... SOMEwhere...in my fingers, to match the tune. On the other hand, WRITING music (i.e., making up a song) I tend to start with a chord structure and then fit the tune/words that are looking for a way out to that structure.