Before you can edit pages, you must join the site and be logged in. Then you will be allowed to edit pages in this Site, by simply clicking on edit button at the bottom of the page. This will open an editor with a toolbar pallette with options.
To create a link to a new page, use syntax: [[[new page name]]] or [[[new page name | text to display]]]. Follow the link (which should have a different color if page does not exist) and create a new page and edit it!
Although creating and editing pages is easy, there are a lot more options that allows creating powerful sites. Please visit the documentation pages to learn more.
For long pages, you may only want to edition one section of the page at a time. Click the "+ options" button at the bottom right of the page to expand this list of options, and click the "edit sections" button to get edit buttons for individual sections.