Board Thread:Support Requests - Getting Started/@comment-26011733-20150826075356/@comment-4018164-20150826112757

Create a new page (can be done easily using the "Contribute" button on the upper-right), and pick a page name that starts with MediaWiki: followed by the name you want to pick your MediaWiki page (e.g. for a mediawiki page named "Potato machine", type MediaWiki:Potato machine).

Before doing so, if that's a custom MediaWiki page, search for its title in Special:AllMessages, to make sure that there isn't an existing system message with that title, to prevent technical conflicts.

However, note that due to security reasons, wikia has disabled editing in the MediaWiki namespace (including creating articles, or moving existing pages to this namespace), except for a handful of whitelisted titles- yet they are not custom MediaWiki pages. So stay tuned and see if they enable editing there any time soon.

In the meantime, you could create, for instance, a project page, and protect it in a way that only admins will be able to edit it.