There's lots of stuff out there about working with voices and plenty of scores on places like IMSLP. However, writing a brief opera or full orchestral work of even 10 or 15 minutes is a lot of work. The good thing about opera is that words tend to go rather slowly, so you don't have to have a large libretto- you can literally buy a pocket libretto of Wagner's Ring Cycle (which is more hours than I have fingers) and it's about the size of a small pocket notebook. The bad thing is that as a result, operas tend to be quite long to allow time to tell the story.
Wagner and Verdi both come from different sides of the opera family, if I recall correctly- I think German Romantic Operas and Italian Operas are different in the way the singers interact and such. I've never studied opera specifically, but I'm pretty sure that division exists.
If I were you, I'd write some short orchestral pieces for full orchestra and then some pieces for choir, and once you are comfortable with the parts independent, start featuring choir and/or solo voices singing words with orchestra to get a feel of how to do it. If you're going with the Wagner approach, you'll want to practice creating and using leitmotifs, or phrases/"chunks" of music that appear and are tied to certain characters, emotions, or scenes. Once you are comfortable with the orchestra, you'd probably want to sketch out your general flow of the piece on piano or something and write the libretto (or find one). I've never written an opera before, but that sounds like the easiest way to start- 'sketching'.
Regarding how to deal with what instruments get what part and harmonic division, that's all part of the field of Orchestration. There are some manuals online that deal with Orchestration (such as the Rimsky-Korsakov one updated by garritan, although it's fairly outdated compared to modern standards), but it's also a subject that can sometimes be best learned by listening and reading scores. I draw a lot of inspiration for my orchestration from listening to different periods of classical music. When you listen to a piece, take it bit by bit and hear what each part is doing in relation to others. Mark harmonies- such as when an oboe and a bassoon might exchange phrases. The best rule of thumb is just splitting up the chords across the instruments according to pitch (effective for string writing esp.)- violin I, violin II, violas, cellos, contrabasses. Orchestration is too large of a subject to really condense into just a few words, but I'd say the biggest thing is just having an ear for harmonies and experimenting until you find a good mix- sometimes unconventional ones can be really gorgeous.
Good luck!