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.