Marketing is the key to e-book sales. There is no quality control for Amazon e-books. It will allow anyone to publish almost anything, and there is no charge to do it. This has resulted in a massive amount of garbage. You could write the greatest book in the world, and it would be so buried that only your friends could find it. Books are listed according to sales rank. So, you have to have pretty good sales to be seen.
A niche market helps. If you have something on a subject with little competition, it's easier to get a good ranking. But, a novel or autobiography is going to be tough. Belonging to a forum and putting a link to the book in your signature helps. If people like you, they may check it out. Giving away free copies to gain reviews and sales rank can help. Amazon allows that for a 2 week period every six months.
The sad truth is that the cream doesn't always rise to the top. As John wrote, you aren't likely to make a fortune. But, even just a couple nice reviews are good for the ego.