I can tell you to spend lots of time writing. To find readers who you trust to read your book and help you make it the best it can be. To read lots of great books. To find and attend a writing conference. To not give up. To remember that the best reason to write is because YOU love doing it.
And, I can refer you to some wonderful links of authors and agents who have said everything I could possibly say, only better. Because they know more and have been doing this for a long time, and doing it well: