I studied orchestration quite a bit as part of my music degree. To be honest, orchestration is going to be really tough to just pick up without other theory training or having played in an orchestra. But fortunately, there is one really good way to do what you are wanting to do, and that is copying. Just listen to a bunch of orchestrated music, whether classical or orchestral metal, and start paying attention to what's going on. Where is the melody? What's accompanying the melody? How do the strings, brass, percussion, and winds interplay? What combinations seem to be most common? Once you can hear these types of things easily, just start applying it to your own melodies and themes.
Another helpful exercise is to just write for a few voices. Maybe arrange your piece for just strings, or just brass, or just 4 voice choir. This will give you a feel for how the melody interacts with harmony and accompaniment.
Finally, I recommend checking out the following. First, is this website: Welcome to the Garritan Interactive PRINCIPLES OF ORCHESTRATION by Rimsky-Korsakov: Northern Sound Source
Rimsky-Korsakov wrote probably the most important work on orchestration. It was written over 100 years ago and is still in use. This website is great because it's interactive, so you can actually hear the things it refers to.
Second, I highly recommend the books on orchestration by Kennon and Piston. I prefer Kennon, but they're both excellent. I don't know your skill level with reading music or other theory-related stuff though, so I don't know how much use you'll get out of these books.
Good luck! Feel free to PM me with any questions. I've played trumpet in quite a few different orchestrated settings, so I'm pretty familiar with it.