If you're a total beginner, I personally think you should leave the idea of a symphony aside for a while (just google "symphony Op.1": nothing), and try pieces with fewer instruments (sonatas, trios...), which you may orchestrate later. After all, Schubert's symphonies were conceived as piano 'sonatas', while Beethoven sketched whole movements in one staff, and so on...
But first, of course, you'll have to begin with short pieces, maybe bare themes of around 16-32 bars...
For the composers you've mentioned, I'll asume you want to write in a tonal common-practise language, in which case you should have swallowed at least 1 harmony treatise (e.g. Piston's, or at least sth. succinct like Rimski's, stay away from Harmonielehre) before trying anything. You should also know the forms in which movements and sections tend to be laid out (a good start: www.teoria.com, wikipedia... for sth meticulous: Caplin's Classical form).
If you already have the right 'tools', then listen and analyze scores! For form and harmonies you don't need the full score, use piano reductions. Then try some of this:
Deduce the form of the movement (sonata form, ternary...), and mark out the sections. Spot all the themes and recurrent motifs. Count the bars of
every phrase, section, everything (proportions, (a)symmetries, (ir)regularities?). Find all the cadences and modulations, and check out where are they (are they structurally meaningful?). Analyze the harmonies (chromatic or modal inflections? treatment of dissonance?) and the movement of the basses. Finally, look how things are developed.
Other stuff:
http://imslp.org/wiki/Musical_Composition_%28Stanford,_Charles_Villiers%29
http://archive.org/stream/largerformsmusi01goetgoog#page/n202/mode/1up haven't read that in full, though
all these Bernstein videos are pretty cook for beginners
... And try to put in practise all what you've learned from time to time.
Later you can learn to orchestrate. Read a treatise... well, maybe you could start with this: http://www.music.indiana.edu/department/composition/isfee/