Be sure you are in visual mode by clicking on the visual tab. Be sure the styling options are available by clicking the show/hide the kitchen sink icon in the formatting toolbar. A second row of icons should be available as well. Highlight the text in the editor you want to make into a heading. Click on the text format dropdown to bring up a list of formatting options. Select heading 2, for example, to see the selected text change. Click on the preview changes button before publishing or the publish/update button to make your changes immediately.