Some of the Project Gutenberg materials are one long file which will cause older readers to not open them and make new readers work more slowly. Generally, it is practical and helpful to make each chapter a separate file.
This can be easily done in either the Calibre editor and in Sigil. You might find Sigil easier to start with as it imposes its own layout and gives you a familiar environment to work in. It also has more developed facilities than the Calibre editor, though the editor is catching up fast.
Sigil has only one drawback and that is that it is only under very slow development and so any problems you encounter (which will be few) are not likely to be addressed.
You can find out general information about HTML and CSS at
www.w3schools.com. They even have a try it out feature so you can alter the example text and see what the result is. It is more aimed at browsers, so there will be things that will work there, but not in ebooks. However, if you are just starting, you are not likely to notice them unless you try to edit pages copied from the web.