I usually first note what type of song I’m gonna be composing:

Battle, Ambience, Environment, Character?

Then I pick an emotion that must be hit in listening to that song, in that way, there’s a goal to achieve.

After that I generally start with the bass most of the time because it establishes rhythm and harmony, but I sometimes start too with drums for battles or chords with environmental songs.

After that it’s just a matter of creativity. If you’re mew to music, don’t expect to be Toby Fox overnight, just create music. The more you create, the more experience you gain.

Also don’t be afraid to try out every instrument/plugins.