If you are allowed to edit pages in this Site, simply click 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!
There is also syntax: [[[category:page name]]]. It creates link with specified name inside a category name space, but displayed link is "page name".
To create a new page from scratch you need only to key in the new "page name" (without spaces) in the adressbar of your browser after the …wikidot.com/___ and hit enter. You will get a "create page" screen with an open link of the new page name.
This technic works for existing pages too - could be you have lost any links to it and you can remember the page name in the url.
Although creating and editing pages is easy, there are a lot more options that allow creating powerful sites. Please visit Documentation pages to learn more.
Tip: on our community-site you can always click on the "src" button on top of the left side menu - this will show you the source of the viewed page!
