Scott is quite right to what he says! Start small and get larger...
The techniques of the instruments, the limits, and the general study of what each one can do and how it sounds, is called "Instrumentation" in one word and is a great chapter and part of the more... generic idea of orchestration.
As for orchestration there is something rather good, for a free offering: http://www.northernsounds.com/forum/forumdisplay.php?f=77
With flash scores, music rendered (very good for what it is) by GPO and a bit of explaining from professor Belkin, a great Canadian composer and friend of a friend (of a friend of a friend of a friend... *ahem*)
As for scores... Try getting the Mozart symphonies (as Scott suggests!) They should be right HERE: http://dme.mozarteum.at/DME/nma/start.php?l=
(along with all of Mozarts scores). And btw, recordings as well, as it appears, although slow: http://www.mozart-archiv.de/
The other excersize that is REALLY useful, but difficult to do alone and one teacher who wrote a book is dead and his book is... extingued in English is Luibowitz (I think is the spelling). Transcribing from piano scores to orchestral. Take a piano score reduced from an orchestra and make it back for orchestra. Then check with the original.
There should be 2 piano editions of various pieces of music, older usually, so you could get ahold of that and start slowly.