It's a tough choice.
If you ask 100 songwriters that question I reckon you will get 100 different answers. I cant speak for all songwriters but I would like to share with you my personal answer to what I feel to be the most commonly asked question in songwriting.
First of all lets look at the definition of what makes up a song. I am an Australian so my songs are registered with an organisation called APRA (Australasian Performing Rights Association) and I have adopted my definition from theirs.
According to APRA (I am sure every other royalty collecting society around the world has a similar definition) a song is comprised of two facets:
Song Words - Lyric of song
Melody - How a lyric of a song is sung.
Everything else around those two things is just the musical arrangement. It took me years to figure out that what defines a song is not the chords you play on guitar but the words and how you sing it.
More often then not if a well known song has been covered by another artist, the arrangement would have been changed but the integrity of the melody and (especially) the words are kept intact. The reason is that the covered song still needs to be recognised by the listener. If the listener sings the words and hums the tune then the song has a better chance of being remembered. Generally the listener doesn't care if there was a Gmaj7 chord in the arrangement or not.
Songwriting and the guitar go hand in hand with me but I always used to write complete arrangements and then try to fit lyrics and a melody to it. I personally feel that its the hardest way of doing things. Nowadays I fit my guitar arrangements around a melody which was inspired by words and I am writing much more than I used to.
The question of whether music or words are written first will never be answered but as songwriters we need not concern ourselves with such trivial matters. Most of the time for me the words (generally a song title in the first instance) come first now and the melody second then the arrangement last however, as soon as I try to make a rule out of this an exception comes along.
Just concentrate on the activity of writing and with the more songs you write the more you will work how you will do things.
Until next time, happy writing,
Corey Stewart
Songwriter/Musician