You must write for at least ten minutes a day. Don't worry if your first writing is more whining than prose. That's good for you. You must get out some of these feelings before the creativity can surface.
A good friend of mine who has been writing for years writes six pages every day. If the six pages come easily, she's out the door to visit friends, volunteer with a literacy program or hunt down antiques. She says, "Some days the six pages are more difficult. But I don't leave, except for regular breaks to refresh my brain, until I'm satisfied." In this way, she's written more than thirty books.
Keep in mind that if you start today and write one page every day for just one year, you'll have a piece of work that is over three-hundred pages. And if you don't? Next year, you'll still be thinking that you want to write.
But what do you write about when writing? If you know with complete assurance that you want to write nonfiction how-to books for children or romance novels for the Christian market, then you have direction. If you're not certain, but know that there's a writer inside who is willing and perhaps screaming to get out, then sample various genres and types of writing. I encourage you, even if you've chosen your specific type of writing, to try other genres. It's fun and it's challenging. Doing so makes you a better writer.