There are a number of self-publishing platforms - Amazon KDP is the biggest, but there is also Draft2Digital, OneBookShelf, and a couple of others. You don't necessarily have to publish exclusively at one single platform. If you publish in print, you will also need to provide a PDF document with the necessary layout (I use #TeXLaTeX for typesetting, but I don't recommend it unless you already have experiences with it from an academic career ant the like), as well as a cover.

You also need public domain works that are available somewhere, preferably as digital download. I am very lucky in that there is an abundance of public domain German-language folk tale collections which I can use, but I don't know what the situation is in Portugese. You could start with searching Archive.org, but if you want to do some serious research, I recommend looking at the citations of modern-day folk tale collections - they often use old sources which might be in the public domain.

(I've discovered a bunch of Italian-language folk tale collections which might be in the public domain this way, but that's probably not much help to you.)