Hi,
A lot of good questions in your post. I myself have started composing about three years ago when I was fourteen. I started by writing simple pieces like menuets, short piano pieces in A-B-A form and other short 36-bar pieces. I never started writing something, however, without knowing what it should turn out to be. If it, in the end, proved not to work out for the particular piece I am working on, I will drop the piece and use the themes for something else, or come back and try to work it out later.
I think you're right that most "real" composers (I'm talking Bach, Haydn, Beethoven, Brahms, etc.) really already had a rough sketch of the complete piece in their head. I don't think they went as far as knowing how much measures the piece would eventually be, but very detailed, yes. An exception, imo, is Mozart who mostly wrote music by means of improvising. He would start a piece by knowing "This is going to be a piano concerto", write down the theme and would write down anything that came down to him. It just so happened that pretty much everything Mozart wrote was perfect already.
I don't think it is appropriate to name your piece according to a certain structure if you didt really know what you were doing whilst writing. Instead, try to make a composing plan before you start writing:
- What kind of pieces is this going to be?
- For what instrument(s) am I going to write this piece?
- How am I going to combine this two into writing a nice piece?
All these questions can be adjusted later on, but you must have some kind of a plan, because else, you wont come any further than writing simple, short pieces which are not structered in a notable way. I don''t recommend getting a teacher in composition just right now. First start writing short pieces in A-B-A form, become familiar with the techniques of different instruments and the figurations they're able to play. Also, you'll become familiar with the structure in music, and you will start to see that basically every piece is build upon certain structures. Sonata-allegro form, for example is basically a large and extended A-B-A form.
Good luck, happy composing and ask anything you want to know. I'll help as far as possible!
BW,
Marijn