1: Read everything you can
Even things you don’t think you’ll like. You need to work out what you enjoy, and why you enjoy it. Take note of the scenes which gave you an emotional or physical reaction—laughter, tears, goosebumps. If you don’t like the book you’re reading, ask yourself what you would change to make it better.
2: Write every day
Even when you don’t feel like it. Try writing in unfamiliar styles, settings, genres. Stretch those muscles. Try NaNoWriMo to get into the habit of bashing out words in a haze of last-minute panic. Write poetry, fifty-word stories, six-word stories, lipograms and pilish. This will build up your vocabulary and strengthen the language centres in your brain.
3: Edit your work
First, focus on the big stuff: character, setting, plot.
Are the characters interesting? Do they do enough? Are their actions consistent? Are they selfless enough to be likeable? Do they feel enough, both emotionally and physically?
Does the plot have suspense? Does it have surprises? Does it hold the reader’s attention by making them curious? Does each event follow naturally from a previous event? Does the ending resolve the main character’s problem?
Does the setting feel real? Are there sensations as well as visual descriptions? Have you wasted too much time describing things the reader will already assume? (The classroom had chairs is a redundant sentence.)
Then the little stuff. Look at each line. Is it clear? Is it repetitive? Can you remove any words without losing information? (Adverbs can usually go.) Can you replace the remaining words with more specific alternatives? There was a car driving quickly down the street can become A Ferrari roared down Henry Avenue.
4: Show your work to people
Rewrite it based on their criticisms. Your ideal reader is well-read, honest, and not too busy to provide detailed feedback. (These people are hard to find. If you don’t know anyone suitable, join your local non-profit writing organisation. I’m with the ACT Writers Centre. Posting your work online is also a good option—try Wattpad.)
5: Approach a literary agent
Your local writing organisation can help you work out which ones are good. I’m with Curtis Brown. Your agent will try to find a publisher for you. Self-publishing can also be a very valuable experience, but don’t expect to make much money out of it. (The people who do are statistically irrelevant.) I’ve used Smashwords before and enjoyed the experience.