A folio is simply a page number. You need to decide on having them at the top of the page or at the bottom. Outside edge or center? In combination with that, you now decide on running heads, which are headers at the top of each page, usually showing ‘Author | Title’, sometimes showing ‘Title | Chapter Title’. Running heads also frequently have the page number on that line, as it’s less page space given over to stuff that’s not the story.
Running heads are relatively new for fiction with the last ten years or so. (Well, they’ve been sporadically used for a long time, but they have become ubiquitous in the last ten years.) They are considered absolutely necessary for reference and textbooks, but less so for narrative-based non-fiction. If you want to have them for your book, have at it – but get some examples to look at before you commit to them. Frequently, they use small caps, italics, bold, or some combination – which means that you need fonts that can support those choices.
About page numbers: when you open a book, the first page is on the right. That’s page one. Page two is on the back of page one, on the left. Don’t mess with that, ever. It’s a tradition you have to adhere to for your book to be taken seriously.
Small digression: the story part of your book is the ‘Main Matter’. The stuff before that (like the title page, dedication, etc.) is the ‘Front Matter’. And after the Main Matter is the ‘Back Matter’. Not very inventive, but that’s how everyone knows what section you’re referring to.
Page numbers begin with small roman numerals (i, ii, iii, iv, etc.) in italic for the Front Matter. When you get to the Main Matter, you start over with Arabic numerals (1, 2, 3, 4, etc.) beginning with the number 1, and starting on the right.
Page numbers and running heads are not shown on blank pages (even though the numbers still increase – they aren’t skipped). They are also not shown on display pages, like the title page and chapter beginnings (and they still count, too.)
Again, don’t mess with this tradition. It is part of the technology of books, and doing it any other way will make people think you don’t know what you’re doing. Running heads? Up to you. Page numbers? Strictly follow the rules, and your readers will be happy.