First of all, realise that it’s very hard, and that writing is a grueling and lonely business and, unless you are extremely lucky, badly paid as well. You had better really, really, really want to do it. Next, you have to write something. Unless you are committed to novel writing exclusively, I suggest that you start out writing for radio. It’s still a relatively easy medium to get into because it pays so badly. But it is a great medium for writers because it relies so much on the imagination.
Douglas Adams on becoming an author, appended to the introduction to The Original Hitchhiker Scripts, as published in The Salmon of Doubt: Hitchhiking the Galaxy One Last Time.