The best introduction to how this site works is from the MediaWiki User Guide. It will tell you how to create and edit pages, upload files and so on.
When first logging on, before doing anything else, you should check your personal information and change your password by clicking the "Preferences" link the top of the screen.
This site uses an add-on to allow you to restrict pages so that only you can see them. This can be useful to track information you do not want other players to see. While editing a page, use the "Restrict" tab to turn this on. You are only allowed to restrict pages that you created.