First off, [Sign on] by using the preferences link to set yourself up a user name. It'll set a cookie so that when you edit pages, your name will appear next to the edit in the RecentChanges? page.
Next, try reading Introduction to Wiki, which should lead you through the basics of how to edit and format pages.
If you'd like some more info on how this particular Wiki software package works, check out the UseModWiki information page.
Once you've done these things, go ahead and start editing pages. Spend some time just looking around and getting a feel for how things are laid out. Use the 'edit text of this page' links to look at how the formatting is set up, how to do links, and so on.
After that, you should be ready to dive in.