A song for my best friend, help me out with something?

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So me being still somewhat of a starter I have learned a few basic chords and I am okay with transitioning through them. I use a acoustic guitar and my question was if I were to write my very close friend a song for her should I write the song first and the music after or the other way around?

And also what would be the best chords for something like Green Day’s Good Riddence song?