I've found some excellent tutorial links to help you get the most out of your kindle or kindle fire but first you need to know a little bit about file formats. I won't go into detail on every e-book format on the web but here are the ones you will see mentioned when you browse this site. (I'll try to add more to this Help page as I answer the questions people ask me from day to day.)
EPUB - There are two kinds of Epub formats you'll need to know about. The DRM protected and the DRM free.
DRM is digital rights management. It's a way publishers have of trying to cut down on piracy. You'll see EPUB DRM protected books from Christian Book Distributors and other book buying sites. If you have a free CBD Reader App you can read these books on the kindle fire, nook, computer, tablet, and other devices. It will not work on an e-ink kindle and it's not legal to "strip" the DRM from the book so I don't suggest trying.
The DRM free books can be converted to a MOBI file to work on any kindle by using a free program called Calibre. Calibre is a program you can use to organize and store all your e-books you've purchased from any website. Here's the conversion tutorial video I found for converting an EPUB to MOBI using Calibre. In the video it suggests you open the file in a kindle app to see that it is converted (at the end) but you can also click on "save to disk" and save the MOBI file on your computer to sideload onto your kindle. There's also a way to sync your kindle to the Calibre program but that's another tutorial for another day. Okay, it's something I've never done but I've never had a need to. When you get good with Calibre you can even convert books in batches to save time.
Once you have a MOBI file you will need to sideload it to your kindle. Here's the sideloading tutorial for an e-ink kindle. It works the same for a kindle fire if you ever need to as well.
Take the time to figure this out and you'll be able to read any e-book from Christian Book Distributors or Smashwords on your kindle fire and you will also be able to read the ones that are DRM free on your regular e-ink kindle. Smashwords lets you pick the file format you want when purchasing and they often have MOBI.