I guess anyplace is ok to start.
That said, personally, I am squeamish about recommending Piston (though I have it and have studied it) or Rimsky-Korsakoff (same) only because they are so old and because I find them relatively idiosyncratic and personal.
By contrast, Adler I find takes a more inclusive approach. No doubt he has his prejudices but I find those lean generally toward the practical side -- what players can actually execute -- and less on the "what I think sounds good" side. I also think Adler's musical examples expose to some extent one of the elements
@JJP emphasized in his post -- the difference of various instruments in various registers. I agree with him that samples often disguise that issue (though oddly not the EWQLSO original library, which I found unerringly predictive of what will sound strained or comfortable for players).
So if you're going down the road of trying to learn orchestration using samples via a DAW or notation program, maybe consider the (I assume very inexpensive now) EWQLSO library as a sketching tool. It also would be far less demanding on computer resources than the new libraries.
What to Look for?
But if you're going to look at scores, be sure to look -- thinking of brass and woodwinds -- at the actual
ranges the composer uses. Every orchestration book worth its salt will talk about the difference in timbre between low, medium and high for wind instruments. It's a huge difference and, for particular effects, the range you want for a particular sound is often surprisingly narrow.
Example: Do you love the trumpet / brass fanfare at the beginning of Star Wars? It's in exactly the register that makes it both playable and sound awesome.
Number One Mistake(s)
The number one mistake is forgetting to let wind players breathe and for the feeling to return to brass players' lips -- rests, in other words.
But maybe even more common and equally pernicious are range blunders. Inexperienced composers often put the French Horns (and trumpets), for example, in their extreme high range for long periods of time. Not only does this disappoint in real life (because it sounds a lot more pinched and strained than it does on samples, and just not as brilliant and alluring as people expect), it's often unplayable, even by professionals. Sure, you can find a sample set with French Horns playing way high, but in real life, with a real budget, and real people those notes sometimes sound and work much better on the trumpet or flugel horn.
By contrast, people forget how brilliant and controlled trombones are in their high register. They can play well up to at least a c-natural in the middle of the treble clef (one octave above middle c; I know the literature has higher notes but I have never written higher than that for a recording gig).
Anyway, this thread offers many good suggestions about learning harmony, studying scores etc. Personally I'm not at all a fan of orchestrating piano pieces -- it's incredibly laborious and, unless you are Ravel reincarnated, often rather disappointing. But I guess one can learn that way too.
Number three mistake? Maybe crowding too much in the low registers? But
@JJP would be a good one to rank common errors and misguided ideas since he orchestrates for others all the time.
Best Study Scores?
There are a lot of scores from which you can learn, but for those seeking to write film music, here is a handful that contains some of what one hears all the time in movie scores:
"Old Guys" Concert Repertoire: Ravel or Debussy -- treasure troves; Respighi; the Russians (Prokofiev, Tchaikovsky, Mussorgsky, Glinka, Kabalevsky, Rimsky-Korsakoff and of course Stravinsky); Richard Strauss; Wagner.
More modern and available, maybe in your library: John Adams -- has a bunch of concert works available (listen to him and then the scores for The Matrix -- hmm); Benjamin Britten; Philip Glass; Lutoslawski; Ligeti;
Movie Scores that are / have been in print: Edward Scissorhands; just about anything from John Williams but Raiders March, Harry Potter, Star Wars, Schindler's List; Back to the Future;
Naturally I'm forgetting some obvious stuff so others please chime in.
Again, just learn
something. One doesn't have to know everything to make plenty of headway.