Write, write, write. Keep a dream journal and use it as inspiration. Read, read, read. Make note of how your favorite authors describe things, create characters, develop plots. And keep another little book just to jot things down that occur to you--ideas, phrases, things you see in day-to-day living that affect you. Take a creative writing course; they not only help you develop skill but help you develop confidence as well.
Study grammar and read the dictionary. No, really.
Also try crossword puzzles or word association exercises; I've often thought of maybe taking five random words a day and trying to come up with five synonyms for each --> Pretty much anything that will expand on current associations in the brain. Not only will it broaden your vocabulary in the long term, but it will also improve your general thinking and concentration ability.
Everyone wants to leap headlong into full literary writing these days, but most people barely have a grasp on the basics.
Yeah, as most have suggested. Just write stuff and get a verdict.
Workshops help too. I was in one once, and it was real helpful. Half the battle I find is coming up with an idea. In a workshop they gave me a writing aim (a title, a specific genre, a word limit etc) It seemed to rub off, and I could conjure up my ideas a lot more clearly.
The worst thing I find is thinking of something fresh, most stuff has already been written, a lot of work out there is just some amalgamation of other ideas, presented in a different way.
I also got a tip about writing structure from a writing agency, in which, they suggested that to open my story I should begin with action - not necassarily guns a blazing, but something should be happening straight off. I'd made the mistake of opening by underlining the situation without writing about what my character was actually doing. When I went back it did look really crap. Although I suppose there is no right or wrong, I guess if it's any good you'll know it, it'll just seem all right.
Anyway, if your interested, here's something you can try. A short story - It must be no longer than 2500 words and must involve these three character types -
1) A vampire
2) A poacher
3) A middle aged man who works in at a bank
Post the story in books and literature. See if people read it. If you want to that is. If not I might have a go. Happy writing.