Well, I believe everyone has a book in them. Fortunately, no one said it had to be a novel.
It could be something as simple as 20 dynamite recipes. Maybe you have insights on managing a coffee shop. Perhaps you know a bunch of fun things to do in your hometown.
Whatever your topic, no matter how big or small, there's probably an audience somewhere. The trick is getting your work to them.
Before you can market an ebook, of course, you need to write one. Any word processor will work for banging out a manuscript. LibreOffice is a good choice if you don't have Microsoft Word. You can also try a distraction-free writing program.
Once you have your manuscript, you'll want to have it edited. You can enlist a friend or family member for this. For longer or more complex works, hire a professional.