I started writing this in paragraphs, and it's grown incoherent and collated (like most of my writing...) so I've condensed it into a few bullet points, but I suggest picking through my full explanation.
I would suggest the following:
- Piano is a great instrument to learn, because you don't need to be past the beginner level for it to help with your composing.
- Learn musical notation so that you have a way to write down melodies in your head and share them with other musicians.
- Know about the staff, time signatures, and basic rhythm and note duration and you can write most basic ideas down.
- Start with monophony, or single, un-accompanied melody.
- Most harmony requires a basic understanding of music theory, and that could bog you down before you even put notes on paper.
- Besides, you can always go back and add harmony to any monophonic piece.
- Musescore is a free piece of software that aids in musical composition, and is worth a look, although a basic understanding of theory is required to not get frustrated by it.
And the full text:
"Music theory" is a super broad category. You can write simple, and even somewhat complex melodies without knowing much of it. "Music notation," however, is more specific, and that's where I'd start.
All of the great composers I know
- Play, or at the very least, know about, at least one instrument
- Know how to notate the songs in their head in at least one form or another
That being said...
Composing is the art of writing down tunes in your head. You can have tunes in your head, and even be able to play them or sing them, and you already have the first step down. The beauty of musical notation is being able to preserve those melodies so that they're never forgotten.
That being said, I would suggest that you have a basic knowledge of the staff (where the pitches are notated in standard musical notation, which I suggest learning because you seem to be interested in orchestral music), time signature (the beat and even, you might say, "chronological domain" of the music) note duration (how much of the "chronological domain" each note takes up).
With these basic tools, you can write down the songs in your head. Now, how you get those songs in your head?
It's a mistake I've made to assume that composing is all about formulas. Of course there are plenty that you can use, but the way that I like to go about it is to pick a melody that pops into your head. Just any random ol' tune that pops into your head. Now, for your purposes, and since it would be easier for you to learn music theory on a need-by-need basis, I would stick to monophony at first (single melody, with no accompaniment). The beauty of this is that you can always return later and pop in chords or counter-melodies in when you feel the need to, but it is satisfying to put the notes down and hear something coherent.
One last argument for learning music notation, is that once a tune is written down, any musician (most musicians) will be able to play it by reading, which means you don't need to do an awkward, "Well, it goes like this..."
Now, as for learning an instrument, I suggest you get a basic knowledge of the piano keyboard, because it is so simple, but it helps tremendously in your understanding of the world of notes. I hardly play piano at all (hardly beyond the 3rd children's book in my studies ;-P), but I can use proper posture/technique to punch out tunes in my head, and visualizing the keyboard helps with a lot of concepts that I would be unable to grasp otherwise.
A great piece of FREE software to aid in your composition is Musescore, although it uses a lot of terminology and requires a basic understanding of music theory. But it is FREE! and therefore worth a look, or a download, as the case may be...