Use the headings provided by your website or software (H1, H2, H3, etc.) You can use paragraph styles in any writing program like Word or Google Docs and headings for websites built on Drupal, WordPress, Cascade, and LibGuides. You can find them in a dropdown labelled "Format," "Normal Text," or similar.
Proper use of headings gives your writing structure: assistive technologies use them to build a table of contents that a user can navigate through. They also make long documents easier to scan. Search engines use the text in headings for discoverability and will give it greater relevance.