In the Article Editor, one of the drop-down menus in the bottom line of the Editor Toolbar is called Format. This allows you to select from a list of pre-defined text formats. Here are examples of the formats you can use. This paragraph uses the Normal format - apart from Headings (see below) and Lists (which do not have a format in this sense), all your text should use Normal format.

This is Heading 1 format

This is Heading 2 format - this format is used for page headings

This is Heading 3 format

Use the Heading formats for various levels of heading. Use the Normal format for all other text (apart from lists).