Answer:
- Harmonization (because studying harmony can help you make dissonances in the right places and resolution as well as what chord combinations are commonly used together)
- Different types of scales and keys, chord inversions, and music theory in general (not necessary, but is definitely a useful tool)
- Instrument ranges (in case it is recorded, the musician can viably play it)
- Types of forms (if you are specifically making a fugue or rondo or sonata)
But you don’t really have to stick to any of this because in composition, anything goes as long as it sounds good! Have fun writing c: