To create new articles simply go to the URL of the would-be article and click the link to create the page.
What is official Mystara is sometimes up to debate, but most of the Mystara community can agree on certain things being canon, and other things being unofficial. Certainly published works by TSR should be considered official, even if they require a note about which edition of D&D they belong under. Official material should go in the default or root namespace.
A namespace is analogous to a folder on a filesystem. Namespaces can be specified as either colons (:) or slashes (/) on this wiki. For example,
http://mystara.us/talk/karameikos
is equivalent to
http://mystara.us/talk:karameikos
Official materials go under the default or root namespace:
http://mystara.us/[article]
Unofficial or Community articles go under the community namespace:
http://mystara.us/community/[article]
Campaign or game materials are acceptable here as well, but should go under the namespace of campaign/[name]:
http://mystara.us/campaign/[your_campaign]/[article]
Unlike wikipedia, article paths and article names are different. Article path refers to the URL, for example:
http://mystara.us/karameikos
However the automatic link to that article will produce the title found on the page: The Grand Duchy of Karameikos. This is very useful because it means you can choose a reasonably universal URL to be permanent, while not needing to "move a page" in order to change the title of it. This keeps links consistent for the future.
The title of the page will be the first heading on that page.