Are we talking full symphony orchestration? Like a full orchestra utilizing all its members (not all at the same time but as needed) and writing things down for each instrument section (and their various members for things like chords, harmonies)? Like following classical theory and writing with those confines or branching out into the work modern composers do?
Orchestration can mean a lot of things... even down to a simple three piece rock band.
If you are talking about symphonic orchestration I'd say study what a classical orchestra is comprised of (all it's units and members). Then study how full orchestra music is written and spread out amongst all those parts (this is HUGE stuff because each musician has their own little bit to play obviously) right down to seating arrangements. Get accustomed to what instruments traditionally do what in an orchestra. Look how various composers create their harmonies, melodies and "leads" (you'll see part of one or multiple instrument sections create/hold/move chords while others of those SAME sections may or may not branch out and be part of the melodies combining with other parts NOT participating in the harmony). Study intervals, inversions, cadences, the range of each instrument and what their particular timbres are useful for/traditional used for.
and so on...
Then figure out what type of compositions you want to create and dig into how similar composers viewed theory. Classical stuff revolves around Major Ionian and then melodic/harmonic minors (you have to understand the differences and where they fall in the step sequence, when they are used, what ALL the possible chords/intervals/triads are on each step... not just Major/minor/7ths, how cadences work with each other to create moods and movement, how timing works and how to write it in notation, etc). You gotta learn all the bizzarro quirks of the keys, notation staff, the roman numeral numbering system that works around the limitations of the Major/Minor system to express chords/harmony (as opposed to modal theory which is much easier to digest).
Really, it's a lot of stuff to learn and keep track of. It requires a LOT of knowledge to write for a full orchestra in the "proper" manner and worth it to take proper courses in. Your local college will likely have courses you can sign up for.
That said... it's all much easier these days (and you can "fake" it) with the proper cumputer programs. One program that gets brought up a lot around here is the Garritan Aria Player stuff which, if I understand it correctly, is capable of full orchestral performances using MIDI and then there are notation software tools (I think Finale is one that gets mentioned a lot).
Sonar... unfortunately, is not the best DAW for this type of work though. I cannot say what is but I think the Mac ones (Logic? Reason?) are what a lot of these guys use and I believe Cubase is supposedly to have really good notation MIDI input.
That way you get to play around with the sounds, concepts and theory involved. These tools are expensive though so expect to spend probably close to $1000 to really get into it... and that does not include lessons/instruction on how to do it correctly/by the book.
Contrast that to how those lunatics hundreds of years ago used to do it though (just a quill and paper and a piano if they were LUCKY... AND they did not have volumes of educational material on the craft they themselves were inventing) and this is all MUCH easier to acheive these days.
It's a laudible goal and I'm most certainly not posting all this to deter you but if you TRULY want to be a composer for orchestral type mayhem then put aside a bunch of time and a bunch of money to get the education and the tools to make it happen.
Knowing how to read notes on the staff and what a 7th chord is just ain't enough... but it's a start.
PS: I am NOT an orchestral composer but would like to be before I die so I've actually researched this stuff. I know a TON about music stuff and the only way I can figure how to become a proper orchestral composer is literally by going to school for it... which I intend to if I ever get the opportunity.
I wish you the best of luck.