Back in 2012, when I knew much less about writing and publishing, I wrote my 320-page memoir Episodes and knew I wanted to publish my first book on Amazon Createspace. I didn't know how to use it, but its Help articles were good enough that I was successful. No one held my hand doing it, although, at the time, I talked to Createspace staff twice who helped me understand it better. The big benefit was the Previewer feature which allowed me to "see" my 6x9 book exactly as it would be printed. Kindle epub was off my radar because at that time, between epub, Mobi, and other formats, I couldn't figure it out.
Today, 2021, it couldn't be easier and you don't have to worry or use epub, Mobi, or anything else other than an MS-Word docx document to upload both as a paper trade book or a Kindle epub book. And I've learned another trick using MS-Word, generating a Table of Contents automatically, making all of that infinitely easier too.
The thing is this: you must write a good book. That's Number One.
Two, all grammar, spelling, accent marks, punctuation, etc. must be correct.
Three, you must format it correctly. In Episodes I had a line space between each paragraph; not good. It broke up the reading. But I thought I knew what I was doing. I didn't.
Four, KDP, the successor to Createspace for both print and ebooks, has fairly good tutorials and instructions to follow. You need some computer savvy, but the steps are there to follow. And you can always go back and correct things and upload a corrected Docx manuscript until you get it perfect.
And Five, it must be perfect. There are too many good writers, millions of books, and something that doesn't read well or look good will not sell. Nor will it make you look good. Above all, you are selling yourself, not just your book. You want readers/buyers to trust you. Trust that you know what you are doing, trust that you write good books.
Using KDP is free because down the road Amazon takes its royalty cut and makes its money off you. I think that's fair for the service. You can sign up for a free account here.
Since 2012 I've learned a ton. I took Episodes off the market and rewrote it completely, shaving 30 pages from it. I just released trade and Kindle versions of Coffee, Cigarettes, Death & Mania, a much more fitting ride with me on the rollercoaster of my life between 1979 and 1996.
I also have a website and a social presence on Instagram, Facebook, Twitter, Linked In, and Pinterest. If you as an author can do those too then you will do better.
Either way, check out KDP for fun, no obligation. Upload your manuscript. and see what it looks like.