Spend some time at mrswizard.com with your main computer. I have been working on this website for years and have written many articles and how-to's that will help you with your general Windows skills (especially file management). These skills are critical if you want to be able to use your Kindle for more than just purchasing books from it on Amazon.
Think of it as an easy to use and understand resource for learning more about your computer. Two of my favorite sections are the Gems and Tiny Treasures. These are favorite software (mostly free) that I use myself and recommend. The difference between Gems and Tiny Treasures are the download sizes. I have gathered some really good stuff over the years. Hope you enjoy them too!
There are many places on the Internet you can download very nice public domain ebooks, but you will need some basic file management, dragging and dropping, copying and pasting and other computer skills to be able to download them and transfer them to your Kindle comfortably.
Before the Kindle, there was Mrs. Wizard's Web. Use it as your free resource to hone your computer skills so you can get more out of your Kindle.