One way to try to figure things out is to download a book from the library here at mobileread.
Open it up in Sigil and look at what items are in the stylesheet under Styles in the book browser to the left. Open up w3schools in your browser and search there for each item you see in in the stylesheet.
You might look at Three Men in a Boat, since it has a lot of formatting that works on many devices. You could just copy and save the stylesheet from it (by right clicking) and add additional files for this stylesheet to the book you are working on. You will need to highlight all the html files in your book and right click to link stylesheets, and tick the stylesheet you just added and ok, to link the stylesheet you just added to your book's files.
Then open Three Men and see in book view where you like something. Then select code view and see how they did it. Then you can try the same thing in your book (since you added Three Men's stylesheet to it, it should work.)
If this sounds like a lot of work, it is going to be some because Sigil is not a word processor. It lets you edit the html that underlies epubs and book view lets you see it and work on some of it in book view.
It might be easier to copy the stylesheet from Three Men and add it to a new blank epub and type a few words. Try applying some of the styles you like to these few words and see how things work. It might be less confusing in the beginning.
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