Go to the MR library and download several books by different uploaders.
Look at them in book view in Sigil and see if you like how it looks.
Where you like the appearance look at it in code view and look at any tags that are shown there, and go to the stylesheet and see what is there.
Like if you see <p class="indent">, go the the stylesheet and look for .noindent and see how they make one particular paragraph not have an indent when all the others do.
If p is listed all by itself, all the stuff following will affect all the paragraphs in the book, etc.
There are several stylesheets in various posts on the forum and in each book. Save one out of one of the books, or copy and paste into your stylesheet and try them out. With Sigil you can eliminate unused ones at the end, so it doesn't matter if you don't use everything. When you have done a few, save a stylesheet aside so you can just start from one having the styles you like.
There is a Nook for PC program, so you can look at your creation at various stages without having to load it into your e-reader. Be sure to close the book in the Nook program before you try to save in Sigil. The program may claim ownership and keep Sigil from saving it.
Please don't feel like your Nook is the only one with issues. Every single e-reader has its own way of interpreting what ought to be a standard. Hitch, a professional book creator has a table full of e-readers just to check out books she and her gang have created. She didn't buy the e-readers entirely for fun, but because the e-readers all have their quirks.