Open a fresh notebook, go to Window > Show Toolbar and notice the pull-down menu that has appeared at the top of the page. Test—writing always in a fresh cell—the effect of each, producing a page with the following appearance:

Title

Subtitle

Subsubtitle

Section

Subsection

Subsubsection

Text

Input

Use Input ONLY TO WRITE COMMANDS, never to write text or comments. As you see, you have, after all, plenty of options!

Inline Formula

Display Formula

New Section

Notice what has happened to the cell brackets, now that we have started a new section.

Now scroll back to the top of the page, go Format > Style Sheet and serially select each of the 16 styles listed, watching to  see its effect. Note that the number and nature of the options listed in the pull-down menu at the top of the page changes as you go from style to style.

Font Manipulation

Select a word, such as this "bigger" (or, for that matter, select an entire cell), hit ⌘ + and get bigger. Do it again and get still bigger.

Similarly, select "smaller," hit ⌘ - and get smaller and still smaller.

⌘ b enters/exits BOLDFACE; ⌘ i enters/exits ITALIC.

Mode Selection from the Keyboard

Sucessively, hit ⌘1, ⌘2, … , ⌘9 while watching the mode selector in the Toolbar. It will become obvious that those keystrokes serve as speedy alternatives to mode-selection from the Toolbar. There are, of course, some modes—in some styles many modes—that cannot be selected in this way.