I came across an interesting quote the other day, I believe it was from E.L. Doctorow (and I'll have to paraphrase it, since I don't remember the exact wording) --
Thinking about a book is not writing. Researching is not writing. Talking about the book you're going to write is not writing. Sitting down and putting the words on paper is writing.That's the main advice I'd give anyone who wants to write. Don't think about it, or plan it, or talk about what you want to write. Just sit down and write. Try to do it every day -- if you can for the same time every day. Try to make yourself write a certain amount each day, even if it's only one page. Don't get up until you've written something, even if you tear it up the next day.
The second lesson is not to think about publication until the story is finished (unless you're already published, or have a contract). Right now, your job is to write. When that job is done, it might be time for the "sell this story" job. It might not; many writers' "first novels" are actually the third or fourth they've completed. Your first work won't be your best; it might not be publishable. But don't put the cart before the horse; don't start dreaming about your book on the bestseller list while you're stuck with plot problems on page twenty-seven. Just write the thing first; then worry about the rest.
Oh and "reading books on writing" is another popular way of avoiding writing. Those books can be useful, but only read them in addition to writing, not instead of writing. Don't let them take away any writing time.