How do I make it so that WordPress displays the post as a heading, followed by a written excerpt?

Simple setting: look at Admin, Settings, Reading. Change the “Blog pages show at most” value (count).

Then look at your Theme Customizations for a setting to show full posts or excerpts. If you don’t put a manual excerpt on the post editing page, then the theme will grab the first xx words (depending on the settings in Theme Customizations). Most themes have a way to specify excerpts or full posts on the home page.