Here are some basic tips for formatting the content within your items:

• Headings: To create headings (and keep things consistent in the site), highlight the text to be made a heading and under Format, select Heading 2 or Heading 3, 4, etc. Try to be consistent and match the heading choices from other pages. Heading 1 is used in the page title automatically so it shouldn't be used again.
• Line break (but not hard return): In HTML, hitting enter will give a hard return which will create a paragraph space between lines. Click Shift and then Enter to get a line break. 
• Bullets or Number Lists: Enter your text with hard returns between the lines you want to bullet point (or number). Select the lines and click the Unordered List or Ordered List icons.
• Bold, Italics, or Underlines: Use Bold and Italics sparingly for impact. Avoid Underlining as it can make the text look like a hyperlink. 

Trying to change the colors, fonts, or design of your site? See this question for information on your site's style.