Using the drop down menu in the content editor you can select Heading 1 through Heading 6.  These headings are styled to match the site template.  Highlight the desired text with the cursor and select an appropriate heading for the text.  Heading 1 is the the largest and Heading 6 is the smallest.

Heading 1

Heading 2

Heading 3

Heading 4

Heading 5

Heading 6