Read a lot. Reading is how you develop critical judgment as a writer. It's also one way, and quite possibly the best way, of learning technique.
...'good' is not necessarily the same thing as 'salable'; you know that your own writing is good, in the same way that you know whether any book is 'good.' Is it interesting? Do you want to know what happens next? Or are you bored, and keep saying to yourself, "What's the matter with this person, why can't they spell?"
The only way to learn to write is to... er... write. This is a truly awful thing to realize, but I'm afraid it's so. You can read all the 'how to' books in the world, attend zillions of workshops and seminars and classes, do tons of research; and none of it counts for anything, unless you get words on paper.