Before you begin

The Special:SpecialPages page is the #1 weapon against unorganized or lacking wiki pages. It can be reached from the main menu cogwheel.
The Special:Version special page has useful information on what extensions are installed for more advanced knowledge on wiki syntax and functionality.
If you're not sure how to use wiki syntax, check out the official Mediawiki Formatting page.
Check out your "Editing" user preferences and make sure you have the enhanced editing toolbar enabled, among other optional tweaks.

Contributing

Place {{outdated}} at the top of a wiki page to display a banner showing that the page is outdated and needs help. View all outdated pages here - you're welcome to help update them.
Special:ShortPages might be looking for more data.
Special:WantedFiles are all linked images that don't have a file attached to them. In 90% of cases, you can let Sargeantmuffinman or TopHatPenguin know about these and he can upload the correct icon directly from source files. (Combined icons won't work for this, such as a human wearing armor, clothing, and holding weapons.)
Special:WantedPages are links to pages that don't exist yet, but someone feels it's needed.
If you upload a file, make sure it doesn't already exist by checking Special:ListFiles. It is crucial that existing files are replaced by uploading a new version - DO NOT upload another version of the file with a different name!
Tables have their own markup that is clean and easy to edit. Visit the official Mediawiki Help:Tables page for more info. There are also a few 3rd-party tools to assist with formatting, such as the Mediawiki Tables Generator.
Templates are extremely powerful wiki additions that help eliminate redundancy in markup and provide a clean, easy-to-edit layout for future contributors. Check out the templates we're using. For example, you can add {{tocright}} to any page to have the Table of Contents move to the right side of the page. Advanced templates can include ParserFunctions (using #if, #ifexp, #switch, etc) for calculation and string handling (e.g. item infoboxes that show a default "Image not found" image if one doesn't exist), leading to some very clean and professional-looking documentation.
Some other features include Magic Words, which can be useful in templates or in special cases, such as hiding the Table of Contents (TOC).
All other Mediawiki-specific Help pages are here if you'd like to learn more.

Cleaning

Special:LonelyPages has a list of pages that might need to be deleted or worked on (anything deleted can be restored).
Special:UnusedFiles might also need to be deleted (anything deleted can be restored).
- Note: normal users are unable to delete pages, but Trusted users can.