Answer: You certainly are raising your hand a lot in class. Good thing, I guess.

I'm a little stumped by this one.

How do you know what you're going to say when you write words? You have something you want to say, and the words express your meaning.

But the meaning of music is not so easily understood. Perhaps you'll have an idea of some sort, but it will be in the form of a melody or a harmony, and then you will put that into notation, develop it, elaborate it, and the process is then like thought itself.

Mozart is said to have been able to compose while playing billiards. He'd put a few in the corner pocket, perhaps whistling inwardly, and meanwhile composed the full harmony in his head. When he got back to his desk he'd write out a finished composition.

Others, like Beethoven, worked out the details with pen in hand. Beethoven's notes show how he repeatedly reworked ideas as he put them down. So he didn't really know entirely what to write when he started: he had perhaps the germ of an idea, and it developed as he worked on it. For example, the first version of the famous melody from the 9th Symphony was not nearly as good as the one he ended up with.

I'd guess, though, that Mozart's mental process was similar to Beethoven, but simply was accomplished without using paper.

Therefore I say: sit down with pen in hand, or at the computer, and just start. Ideas come as you work. Don't wait to know.