What can you do to get started? Having recently been in this exact same position, I would suggest that any author do these top ten things (in any order, after #1):
1. Pick a pen name. Your name will be your brand, so go with what you like best.
2. Set up a web site. Have a basic paragraph about you, a photo when you're ready.
3. Set up a blog connected to your website (I have my blog as the main page of my website). Begin blogging about writing-connected stuff. I do a full-on book blog, with lots of posts about other people's books. Many of those I get from book-tour companies; that means I can have a new post just about every day. And I post my own stuff (reviews, lists of top-five-favorites, character interviews, and so on) about once a week.
4. Set up a Facebook author page.
5. Set up an official Twitter account.
6. Set up any other social media accounts you want to use (Pinterest, Instagram, and the like).
7. Hook all of these accounts together by using something like Hootsuite.com (there are others, but that's what I use) so that everything gets cross-posted everywhere. Once you start doing marketing (much of which will be up to you by necessity--you are more invested in your work than anyone else!), you will find that this saves you time, which will become increasingly precious.
8. Begin practicing using all of these before you actually publish a book. You're going to want to say something other than "buy my book!" so it's a good idea to start using the accounts before that's all you want to say. :)
9. If you are self-publishing (or publishing with someone who will let you have input on this), begin looking at Amazon for category keywords for your book. You will probably sell the book on other sites, too, of course, but Amazon is still the biggest seller of books, so it makes sense categorize it according to Amazon's lists. Start looking at similar-ish books and seeing what categories they're in. (You'll see something like this: Books > Science Fiction & Fantasy > Fantasy > Paranormal )
10. Keep working on writing; remember, none of this is any good without books to go with it!
Authors and writers: What other suggestions would you give authors with forthcoming books about beginning to work on their brands?