From here, you can set the name and URL of the custom section. Once you've created the section (under the Add New Section area on that page), you can View Page to access it. There will be an option to Edit Page, which will bring up a WYSIWYG menu you can use to create custom content that isn't already baked into the system.
After you have created a custom section this article will help you add it to the main navigation bar of your site. Article