If you did step 2 - you can visit your website immediately:
<your-account-name>.github.io
For now you can see, the websites name and description. Your markdown file was converted to HTML by GitHub, which your browser can read.
add another Markdown file an link it in both files like so:
### | [Home](./README.md) | [Subpage](./subpage.md) |
GitHub Pages offers the possibility to add a theme to your Markdown files.
Got to your_repository > Settings > Theme Chooser