Learning orchestration is something that one may call "a life goal". You may very well try, and try as many times as you want and still not produce "adequate" (whatever this may mean) results...
The point I'm making is that with a thread like this, you won't get very far. I guess I could give you some pointers, but still it wouldn't be enough...
In any case, just so you don't say I didn't try, here's some general advice:
1. Depending on the work you're working on, you will be changing the orchestra to use. For a nocturne, where you want a Debussy effect, you might go for more harps and lack of too many percussions. In this case and a March, I think the case calls for many brass instruments and percussion.
2. The exact size of the orchestra, in regards to the strings is usually not too specific. Meaning: Unless you have a very strong reason to request 12 violas, you'll leave it at "whatever" and know that any orchestra will have approximately the right combination of each string section. This doesn't mean that you won't take into account your other instruments: if you're using 8 French Horns, obviously "just" 12 1st vlns, won't cut it!
3. In your score, and all your compositions you will need to take into account what can or can't be done for each instrument. You're a pianist, so this comes natural when working for piano, but if you go for a wind instrument, as opposed to violin (that you've tried), chances are that you'll forget that a human needs to breath!
Then there's all kinds of quirks for every instrument, which needs to be taken into account.
So we come into learning about orchestration.
4. The first thing you need to do is to become acquainted with the orchestral instruments. See what they can, are comfortable and can't do. What they like and what they don't. Check on what a usual orchestral writing for a violin looks like! It's certainly not the same as a solo violin. Same goes for every other instrument. Check their ranges. See how loud they can sound on the lower register, as opposed to the highest register. Check if the bassoon (for example) can sound soft in its lowest notes, or if the trombone can do a perfect 5th gliss (naturally it can't, at least not by using the valve alone...). See how the instruments work, and why there's the transposed instruments in the first place (why on earth is the Clarinet in Bb and not in C to get it over with).
5. Then see how they combine together, few and few, or more. What does an orchestral hit looks like in score, why are the violins entered a tiny bit early, or why it's quite often to see vlc + bsn together. Also learn all the abbreviations of the instruments, and their techniques. sul pont., pizz., ftg. etc...
6. And try. Keep trying. Start with a normal size classical orchestra and do some Mozart, then some Haydn, then some Mendelssohn, perhaps some Beethoven, etc. Start small. Can you transcribe one of your works for string quartet? Transcribing and orchestrating is not just "place the right instrument in the right time", or "divide the voices into instruments and you're done".
7. ANALYSE. Check what Beethoven did, what Shostakovich did, what Mahler did, how did Ligetti do his micropolyphony? Also check Williams (Star wars, etc...), check John Barry, check every composer you can think of and any work you can get your hands on the score.
There's a wealth of links available all over the place. Try northernsounds.com for a tutorial on the Korsakoff orchestration (old and outdated, but still valuable for a free tool). Check the philharmonia orchestra (I think) which has free samples of the instruments. Check your local library for scores and spend countless hours to study scores.
And THEN try to orchestrate your March! (I'm kidding here... Go ahead and try, but I was unable to give you some direct instructions. Instead I gave you a four year plan!)
