The most important thing in fiction writing is a compelling story. There are a number of ways to achieve that--character, plot, writing, etc--but it's the most important thing.
And no one can tell you how to do it.
Seriously. No class, no workshop, no blog by any agent (no matter how awesome) can make your writing compelling to others. It is, or it isn't. Period.
But what those other things can do is help you avoid mistakes that everyone makes when they aren't thinking. J.A. Konrath judged a short story contest back in June, and he's got some guidelines about how not to write a story. His list is all examples of things that writers do that slow a story down or make it feel like every other story out there. Prologues, dreams, a character waking up, weather...all things that happen over and over and over again, and are almost never a good idea.
My favorite? "Do Not Start A Story With A Setting Description." The funny thing about this edict is that one of my most favorite books in the world is East of Eden by John Steinbeck, which starts with basically a whole chapter of setting description about the Salinas Valley, where the story takes place.
That's the point, really--you avoid these mistakes until you're John Steinbeck, and you don't have to anymore. And even if you become John Steinbeck, you still don't need to start with a description of setting; I always skip that chapter anyways. :)