you know, frankly, when I first started composing, some of the very first things I wrote were pieces for a full blown concert band, as well as some chamber music. I wrote whatever I wanted, and I asked people for feedback, edited what I can, and that is how I learned orchestration, along with listening to practically everything. I completely agree with Austenite in that that in order to write a symphony, you must have listened to hundreds and hundreds of different symphonies from all different eras. But I don't quite agree with the notion that if you want to learn how to compose, start with a piano sonata, then a string quartet, then add one instrument at a time. Everyone learns differently, so don't be afraid to attempt to write a piece for such a large ensemble, I can almost guarantee you it will be terrible, but if you learn the most by just doing it all at once, then do it. But I do not recommend writing a full blown symphony, that's just about too much for someone like you to handle, heck, it's too much for me to handle! As far as learning how to write a symphony though, which was your question, listening can be your friend, try to figure out about each composer you listen to, what they were trying to say, what their strengths are, take note of any weaknesses, figure out the form, is it ABA, Rondo, which is ABACABA or something like that where it always go back to A, or Sonata form.
And just a very brief, and no where near detailed enough description of Sonata form
you start with the primary theme, then transition into the secondary theme to contrast the primary theme by putting it in the dominant key (for example if the primary theme is in C, the secondary theme is in G), that's gonna be your exposition, then you develop your themes this is your development , then you get to the recapitulation, where you recap on the exposition. You play your primary theme, and secondary theme, but your secondary theme is in I, not V this time (C, not G). This gets rid of the tension between the two conflicting themes, and states I as the victor over V. Then, you finish any tension left over in the coda. So, here's a somewhat chart to make it easier
Exposition: A (I) ------(transitional theme)- B(V) ---------(transition)|Development: A/B (?) ------------|Recapitulation: A(I) -------(transitional theme)- B(I) -------| Coda: End it!
Hope this helps! Also, some other symphony suggestions, these are a bit different because they tell a story, which is perfectly fine to do in a Symphony if that's what you want:
Berlioz: Symphonie Fantastiqe
Messiaen: Turangalila Symphony
Honestly though, composers have screwed up symphonies so much until now, the only thing all symphonies would have in common would be multi-movement big piece.