You can use "bold" for headers above paragraphs but it's not recommended. Instead, use the HTML "H" tags such as H1, H2, and H3. For your needs, it is recommended that you stick to H2 only when editing text.
1. Select the text (This is typically a few words above a paragraph)
2. Click on the dropdown that says "Paragraph" (below the Bold and Italic buttons)
3. Select Heading 2.
Note
- Sometimes doing this will make the entire paragraph into an H2. Don't panic! Just add a space, select the paragraph text and choose "paragraph" from the drop-down list.