Provide structure to your documents by using headings. All users benefit from visual and logical structure, but this is crucial for users of assistive technologies.
There are six levels of Headings. Your Heading 1 should be primarily used for the title of the document you are creating. Subsequent headings should then be used hierarchically and nested sequentially. You should not skip heading levels, such as using a Heading 4 directly below a Heading 2. You may have multiple heading 2s, 3s, and so on, so long as the items have a similar level of structural importance (e.g., a heading 2 could be chapter 1, chapter 2, chapter 3, but sections in each chapter might be heading 3s).
When possible try to adjust the styling of your headers so that they appear larger and more distinct than the surrounding text.
The following picture shows how you should be using headings within any documents you create.
For more information, See WebAIM's page on Headings.