Filling in content is relatively easy. Find a page lacking in content (
these two lists help), click 'edit', and fill in what you know. If you don't know anything but want the content filled in, put {{stub}} on the page (for general content), or put a request on the relevant Talk page (for specific content).
Templates are easy to apply, but harder to write (though copying other wikis' templates is possible). These are what give the pages a uniform appearance, and fancier stuff can be hidden inside of them.
Variables can be used with templates to help automate some of the uniformity (this is what H&H wiki does to simplify the process of making images show up).
Parser functions help further automate the formatting of content and enable it to do some simple math and logic checks.
Semantic MediaWiki lets the wiki behave more like a relational database.
If you're interested in learning some of the more advanced stuff, I'd recommend following the order I outlined above.
Wikia Help,
Wikipedia,
MediaWiki, and
Meta-Wiki also have a lot of relevant content.