Unfortunately, the composer must know everything, so it seems. Orchestration, extended instrumental techniques, a knowledge of styles past and present, acoustics, notation, form and structure, electronics, etc. Melody, harmony, rhythm, tonal systems. The list goes on.
Fortunately, you don't need to know all of these things before you start writing. You can start composing, and discover these elements as it comes up. Some things may not come up for a good amount of time. The hard part is not to let all of this paralyze you.