The easiest way to add a Heading would be to select either H or H2 from the tool bar at the base of the note.
But, if you already have text that you would like to make a header, you can highlight the text and select Heading from the drop down. .
You'll notice that we have 2 sizes of Headings, for comparison here are the two sizes:
You can also add in Headings by clicking the slash command to open up the slash menu. You can also choose to type in "heading" to find those options faster instead of having to scroll in the menu.