Your audience determines not only how you should write, but what you should write as well. To assess what your audience is looking for, the best place to start is with the genre you want to write in. For example, my favorite genre is historical mystery fiction, which, thankfully, has quite a few predecessors, such as Ellis Peters. Now, I have read all but a few of Ellis Peters' Brother Cadfael series; I saved a few to read in my dotage. There are other writers, such as Agatha Christie, who wrote some historical mysteries, and it's important for me to know as much as I can about that market if I wish to succeed in it. That means, in one word: research.
I have to read as many historical mysteries as I can get my hands on, not only to see what doesn't work, but also to read what does. Recently, this particular market has started to heat up, so there are more new entries to read. You should be doing the same with your market of choice; read everything you can get your hands on, so that you understand what your audience is buying and what they want to see. This audience awareness will help you sell your book to your agent, and it will help hone your focus as you write.