What Do You Think Is The Most Important Thing That A Composer Must Know Before Embarking On Composition?

To have something to say, or your music won't go beyond a few bars.

This.

Do more with less, not less with more.

Get practical. Prepare for an investment in time, labor, emotion. Prepare not to feel "rosy" all the time. Work through that. Pretend that you're a weight lifter. How does HE prepare? His goal is moving a huge weight with his muscles from the floor to the air. The composer is focused on moving a note through time with his brain. Though sometimes it feels like lifting weights.

I've been reading through a biography of Brahms and came by a quote I thought appropriate for this thread:

'Another secret of [brahms'] consistently high average of quality was his contempt for mere contrapuntal ability, for cleverness in itself. "I send you the same lot of canons again," he wrote to Joachim, the crony with whom he had agreed to exchange weekly counterpoint exercises and criticism, and delinquency to be fined one thaler. "Apart from the scientific side, is it good music? Does the ingenuity lend it more beauty and value?"

'"Is it good music?" All his long life he insisted with growing tenacity upon an honest answer to this query. And at the first suspicion of a negative, open flew the furnace door!' ('The Unknown Brahms', 1936, by Robert Haven Schauffler, pgs 169-170.)

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.

Being able to play at least one instrument to near-professional standard. There has been no great composer who was not also a capable performer.

Found a quote in Gartenberg's 'Mahler', pgs 299-300:

'Blaukopf points out another significant development beginning with the Fifth Symphony; viz., Mahler worked with orchestral conception from the outset of the work, doing away with the piano, on which he had previously worked out initial conceptions (a habit which most composers maintain to this day). Mahler advised even Marshalk not to compose "from the piano". This remark is fascinating because another composer - in another field and practically another world, although he was a contemporary of Mahler - had expressed the same thought. Johann Strauss [iI], who shared with Brahms the habit of composing at a stand-up desk, shared with Mahler a disinclination to compose at the piano. ("I never compose at the piano. The piano has the habit of making you do what it wants you to do.") Mahler's emancipation from the piano changed twentieth-century orchestration habits, because musical imagery no longer had to be transposed from the keyboard into the orchestra.'

I don't think it's necessary at all to become a master of any certain instrument. Especially after Mahler, if Garternberg's mancrushy biography shows anything, knowing what an orchestra can do, the limits and possibilities all the various instruments, is enough to compose great things - along with theory, etc. An instrument might come in handy if at first you don't have a starting sound in your head, but with today's tech - sound libraries - it almost seems unnecessary unless you plan on being some kind of concert-player or member of an orchestra.