Theme/Genre
Pick an overall theme/genre
Pick the instruments/sounds you want to use. Listen to lots of songs from your chosen genre – what kinds of sounds do they use?
Pick the tempo (speed) of the song. You might change this later on if you find that the song feels better a little faster or slower. Usually the tempo stays the same for the length of a song, but sometimes there is a little ‘push and pull’ (getting slightly faster or slower in different parts of the song if it feels right).
Decide on the main rhythm of the song. You might want to search up some drum beats for the genre you’re writing in.
Write different fills to vary up the verse and the chorus.
Decide on notes for the verse and chorus for piano, bass and guitar (or whatever instruments you’re using in your song).
Fit lyrics in with the music.
Tweak the music and/or the lyrics to make the song better.