Now that your book is in order, you can publish it through Kindle Direct Publishing. The entire process is guided and quite simple. Sign up is free, and a new publication is usually for sale within 1-2 days.
With Kindle Direct, you also have the choice of using the Kindle Select program. With the Kindle Select program, you agree to only sell your eBook through Amazon for a three-month period. In exchange, you are given a few promotional tools and often better visibility thanks to them.
After this three-month period, you may choose to opt-in again or expand your distribution by going to other retailers like Apple, Nook, and Kobo.
As suggested before, Smashwords is a good solution for all of the other eBook retailers. You will have to make sure your book file follows their rigorous formatting guide, but if you can master this formatting guide, you’ll be able to produce great quality Kindle eBooks as well. See that here: