This is always an exercise where we struggle. I think this link is more aimed at nonfiction, but some of the questions are still relevant for fiction.

And if you want to get started on your author platform, here are a few ideas.
http://www.authormedia.com/89-book-marketing-ideas-that-will-change-your-life/

Here's an example:
http://www.advancedfictionwriting.com/blog/2012/07/11/defining-the-target-audien...

Good luck!