Which is more important: chords or melody?
Which do you write first: the chord line or the melody line?
There's no right or wrong way to build your song. But as a rule of thumb, in rock music the chord line is more important and is written first, and the melody line is written second to coordinate with the chord line.
In classical music, the melody line was more important and was written first. The chord line was written second to support and lend structure to the melody line. (And, as I mentioned in yesterday's post, the chords were mainly [one] and [five]7).
(Note that I'm talking narrowly about the Haydn-Mozart-Beethoven classical tradition. The classical section of a record store includes a rich variety of traditions and genres, as diverse as the rest of the store put together, so it's hard to make any general statement that applies to all of that different music.)
You can easily hear the difference in these two approaches. In the melody-first approach, the chord rhythm is irregular. The chords can move quickly and arbitrarily to support the whims of the melody.
In the chord-first approach, the chords usually have a simple, neatly structured rhythm. Often it's as simple as one chord per bar. But when the chords have a more complex rhythm, it has its own internal logic, and it's not dictated by the shape of the melody.
Sometime in the middle of the 20th century, songs broadly switched from the melody-first approach to the chord-first approach.
If you want to evoke an older style, or if you just want to do something different, try the melody-first approach. Write a melody that makes musical sense even without any accompaniment. Craft your entire vocal melody before you start thinking about chords.