The answer to this depends a lot, IMHO, on what your project is, and what your short- and long-term marketing plan is. Different genres are marketed differently; self-publishing is very different than traditional publishing, etc.
I'd look in two directions. One is, could you write some short stories as a side project. Nothing advertises writing like more writing, and getting some short stories up for sale can start getting your name out, establishing credentials, pointing readers towards your upcoming work. Even very short pieces can be useful; hold 'em in reserve until you have somewhere useful to put them.
Secondly, you can start considering how you'd like to market and brand yourself. Basically, you want to ask yourself how you'd like people to view your book, and you as an author. Who will your readers be, and how will they find your book? Will they find it at an airport bookstand, or will they hear about it on indie science fiction review blogs, or will they find it when they're searching Amazon for a low-priced light read in their favorite genre? These all require different approaches, and different levels of visibility/presence of your author-persona.
If you expect your marketing to have a large online element, then you can start thinking about what that element will be. If you want to be, say,
a wise and wisecracking author-figure, or if you want to share parts of your life and worldview and sense of humor with your readers in a personal blog, then that's the kind of content that's good to start well in advance. That way, when you're ready to promote, you've got an established blog, not an "under construction" page. But if that's not going to be your main direction, then putting a lot of effort into a blog or webpage now is likely to be unnecessary.
Whichever direction you choose, best of luck!
