See Mediawiki's help page here:
Screencast on editing the wiki
If you haven't edited a MediaWiki site before (this wiki is a MediaWiki site, just like Wikepedia) then watch Ian's short screencast that shows you how to login and edit the community page.
You may also want to read the MediaWiki Formatting page to get an idea of all the markup commands that are available.
About the BuildBrighton mediawiki skin
Header image from gmacklin on Flickr