Well, you're already a writer.
Becoming an author . . . is a little harder. Hehe, I'll let you know when I've figured it out myself. >.< But mostly it's about sending submissions to publishers, getting rejected 900 times and then hopefully getting that one big hit that gets turned into a movie or, if you're really lucky like J.K. Rowling, a franchise. These days, there are several self-publishing tools too, like publishing e-books or publishing through amazon.com or Barnes and Noble. There are good reference books that list publishers, like this one:
You just can't be afraid to put your work out there and you can't be afraid of rejection. Kathryn Stockett, who wrote The Help (movie being released in theaters this week) was rejected 90 times before getting her book published and now look at her.