Again? You spent all this money, charging up vulgar debt only to forget how to use this software? You’ve been intimidated and captivated by Mathematica since the mid-1980s. You have to turn this computational environment into a familiar mental landscape! This is a tool that makes documents, living documents of computational data with human commentary. This tool should be as familiar as Microsoft Word is to you—it will be.
The page layout features of Mathematica are limited (cells are stacked vertically and cannot be laid out side by side in multiple columns which must be why notebooks are often displayed by default in narrow windows)—but they should be reviewed first because we are making human-readable documents here. First of all, this sentence should be the second paragraph in the second cell of this document (enter Ctrl+Shift+e to invoke Cell > Show Expression to see the StyleBox TextData of the Cell—and that everything in a notebook is a Wolfram Language Expression). You started writing this document by using the menu (since you have forgotten the keyboard commands). You selected Window > Toolbar > Formatting to show the Formatting Toolbar which will allow you to see what style is in use as you move the up/down arrow keys to move among the cells of this document. You also selected Format > Stylesheet > ReverseColor to get a dark background (easier on the eyes but will drain the ink from a printer so beware!).
You are generating new paragraphs by hitting the enter key within this second cell. You are not sure whether these are line breaks or paragraph breaks because there appears to be no line spacing expected from, say, Microsoft Word paragraph breaks. By convention, you are using Ctrl+i for italics and Ctrl+b for bold and these commands are working as expected. (You also noticed the Ctrl+. command from Edit > Extend Selection menu item. This allows you to select cells and cut (with Ctrl+x) or delete them.) The italics/bold keyboard commands appear as tooltips in the Writing Assistant palette under the Writing and Formatting pane. This assistant does not show any paragraph-spacing values. You do see this after searching for ParagraphSpacing in the Option Inspector (available under the Format menu, making sure that the first dropdown is set to Global Preferences).
The keyboard command to make hyperlinks is Shift+Ctrl+h. You notice here in Mathematica 11 that inserting a hyperlink makes the font size increase. You might use the Alt+- command to decrease the size but the Formatting Toolbar shows you that the style is Chapter for hyperlinks by default (probably because hyperlinks are expected to be used for a table of contents more often)—change the style to Text with the hyperlink selected to get the correct font size. Now that you know how to make hyperlinks, you can list the YouTube videos with all the tutorials you have seen and forgotten about: