Read the lessons on CSS and xHTML at https://www.w3schools.com (the site you referenced).
Note that you can duplicate a default theme folder (start with all new files locally) and name it something new and modify the comment on the style sheet (to reference your info, not the default theme info). It’s very important that this information be unique as you cannot have duplicate themes. Upload that folder and use your admin to select that theme for display.
Then mess around with the files in THAT theme folder, confident you have a fresh backup of untouched files. Note that all of this assumes you have FTP access to your site.
Then, I’d go into my user profile and TURN OFF the wsiwyg editor option… just an opinion, but you won’t learn anything from letting some code do the work for you.
Finally, when you DO understand and are having success with your post formatting and template file editing, you can think about what tools you really need for post/page formatting and make your own by editing the /wp-includes/js/quicktags.js file…
Note that modifying core files is frowned on here… but this is one place I make an exception because none of the plugins that claim to do this are very good. I just know that when I back up my site files for upgrade to keep a copy of that file… I prefer conscious formatting of posts/pages content — headings, paragraphs, lists, etc — through the quicktags.js file, you can edit how the existing tools work (for example, the existing list buttons indent and add new lines and other crazy stuff that I simply remove via the quicktags.js file).
Again, I want to encourage you to experiement inside a UNIQUE theme folder and starting with a copy of the default is the best way to do that (or classic)… NOT with some third party theme. ALSO, be careful and don’t mess around with the core files until you REALLY understand what you are doing with CSS, xhtml, jquery/js and PHP.
Good luck with it!