Amazon, Amazon. Truly, the 900-lb. gorilla of the ebook world, and, as of this writing, not likely to be dethroned anytime soon. So, how do you upload to the KDP? Where do you go, and what do you do?
Places to Find Help At Amazon:
They used to have this really cute video, that I thought was more helpful than all the text, but, alas, that seems to have gone the way of the quill pen. They do have a pretty awesome site-map/FAQ, which is a hotbed of quick-links to get you right to the question you want answered: Amazon Help Quick-Links
I personally like the step-by-step instructions better, for the first-timer: Getting Started With Amazon Self-Publishing
And then you can login at the KDP (Kindle Digital Platform) yourself: https://kdp.amazon.com/self-publishing/signin
Of course: if you run into trouble, you can EMAIL AMAZON for help. How? Go to your KDP page. Even if you don't have an account, just click on this link: Help! I've Authored And I Can't Give Up! and scroll down. On the LEFT-hand side, at/near the bottom, see that yellow button? Says, "Contact Us"? Just click that. On the next page, they have some topics you can choose from, but they also have a handy contact form. Simply fill it out, and send the message. Just in case you're not logged in when you want to send it, make sure that you include the email address you've used on your Publisher's Account. Amazon is uber-friendly, and they'll get back to you amazingly fast!