To add a post to the homepage, go into the _posts folder and add a new file. The filename should follow the format
year-month-day-title.md
To the top of this markdown file, add the frontmatter text
---
layout: post
title: [Add title]
author: [Add name(s)]
---
This text tells Jekyll that it's a post so it knows how to format it.
When writing a post, by default the first paragraph of text is what shows on the homepage, before the READ MORE link. I think for posts, best practice should be to have a one or two sentence lede as the first paragraph, so that will be what shows on the homepage.
Editing any one of the non-post pages just requires editing the corresponding markdown file in the root directory. Just make sure to leave the frontmatter text alone.