Hey, welcome then I guess. Like anything it’s all a gradual learning process/curve. My very first attempt at trying to write for orchestra (being a long haired Metallica playing guitarist at the time) was 1 flute, 1st violins, 2nd violins and nothing else because I had no studies, education or most importantly no practice . Each composition I attempted to use some new instruments.
Sounds good so far, the themes develop nicely and sounds like you’re already experimenting with what instruments to use so just keep trying out different instruments combinations. I liked the sound of the accordion, I think it’d be better to substitute the flute for a clarinet and use the accordion more. The Tantalus theme below is similar and might give you some ideas.
I liked the tune, reminded me of a mixture of; http://www.youtube.com/watch?v=bxmgjddikwk and http://www.youtube.com/watch?v=bEzLoQG7Y2I from Final Fantasy 9.
The pitch down part sounded very cool. The vocal instantly sticks out as a sore thumb due to it being completely to the left channel. In my opinion either bring more focus to the vocal throughout the piece and have it central or drop it altogether. It sounded good, weirdly interesting, but it’s not featured enough to warrant it being there at all. I think if you brought it to the center and used it a bit more it’d sound really cool.
As for getting used to samples, just takes reading the manuals and practicing. Different sample libraries respond differently to the various CC data so there isn’t one rule for them all. Depends on your budget and how realistic you want the samples to sound as to which libraries you should purchase and which you’ll enjoy using the most because enjoyment is the most important thing.
The more pieces you write and strive for perfection in every aspect the closer you’ll get to your own personal goals and that’s all that matters, achieving your goals (In regards to sample libraries the use of the word perfection to me personally means perfecting all the imperfections that make it sound more like a human performance.)