Q: i want to write a nice song. what instrument do i do first
A: use an instrument that can do chords and also be slightly rhythmic, and use your voice at the same time. Use these to write the song. If you want a recording of the song with more instruments, then i would recommend doing percussion first, assuming the song is entirely written already which would have been done on the chordal instrument with your voice, and plenty of descriptive instructions as well