Well, dragonworks is right too... Theory is nice, but you need alot of theory before you can start. Just go for it, buy the books anyway, and read them when you don't have inspiration...
Jazz progressions aren't always that more difficult... After romanticism, with impressionism and expressionism, there was alot more freedom. Composing in modes, to even atonal. But it's very advanced music theory.
Jazz also has it's harmony, but the nice thing there is, they start with easy progressions, and just keep reharmonizing them untill they got something *evil*...

But mostly, if you know it, you can still play over the easy progression.

The book by levine is a good one on this. More than half a book about reharmonizing. Just think what you can do with your orchestration, if you can have them play different progressions over the same tune, some time...
And please stop kicking the classical theory-focused me... Jazz is music to play, not compose. I cannot play...
