Your writing is YOU. Just as silkworms spin silk from mulberry leaves, you spin writing from your life's experiences. The more you know and understand yourself, the more you will know and understand the world, and the more you'll have to write about.
I'm a believer in the "black box" theory of personality. That is, you know someone (and yourself), by their actions -- by what they do -- NOT by what they say, or by what they think.
This theory comes in handy when you're writing fiction. :-) The curse of the neophyte novelist is the thinking and talking head. If you decide that your characters have to DO stuff, you'll find that your fiction is suddenly much more interesting.
To begin to get a handle on knowing yourself, commit to a writing log, and/ or a journal.
A log is usually project-specific; that is, you would keep a log for a novel, or nonfiction book. However, you could also keep a log for your goals. Many years ago, I kept a Goals Log, and found that it kept me focused and working steadily, so I've decided to start keeping a Goals Log again.
Your journal is your day-book. It's a place where you can talk to yourself. One of the benefits of a journal is that it gets all the whining and complaining out of your head and onto the page. When you see it there, it starts to look boring. Then you can use your journal to develop material.
For example, you might go to a coffee shop just to people-watch. That girl with all the piercings and tattoos: maybe she could appear in your novel. Or maybe you could write an article on body decoration. Unless you're keeping a journal, you've lost the idea of that girl as a character, or as the impetus to an article, within a few minutes of seeing her.
