The first thing that people should take a look at is the basic editing page. This is straight from the people who built the wiki, and has a wealth of nice editing stuff. The purpose of this page is to summarize some of the basic things that people might want to do.
As a side note, when editing pages there is the option to enter a name as the Author for the edit. Please do this so we can keep track of who's doing what. Use your character name too please. We may roll back changes that haven't been signed.
Not being a master editor myself, if people want to help out by adding stuff they are welcome. Most of the things that can be done with pmwiki are probably well beyond what we really need. One thing to keep in mind that a really good way to learn how to edit pages (i.e. what sort of markup to use, etc) is to look at the wiki source for different pages. Thus, just click the edit button on some page and take a look at that. You probably won't be able to see it for guilds you don't have the password to edit, but you can ask around. If you check out the pages at the pmwiki website then you can find some more ideas.
If you want to experiment with something before throwing it in a guild page, you can go play in the WikiSandbox.