To help get you started there are a few very basic things to get you started.
First, you will need to get an account to be able to edit any pages on the wiki. Then the easist thing to do is take a look around and if you see something that needs fixing select the "Edit this Page" link that appears on the page. (It's location changes depending on the Skin you are using for the wiki. In the default skin, the "edit this page" link is located as a tab at the top of the page.)
When editing a page, you should try to use the wiki syntax. The syntax is, generally, pretty easy for basic stuff (headers, links, text formatting) and a little bit harder for the more advanced stuff (images, tables, formulas, etc...). The best way to see how something is done, find a page that has something you want to do and click "edit this page". Take a look a the text that was used to draw the current page and try it out on the page you want to edit. Use the Show preview button at the bottom of the edit page to see what your changes look like (since the page will not be saved yet.) If you want to cancel your edit, just select the cancel link and no changes will be saved. Use the Save page button to save your changes. If you just want to try something out, go ahead and edit the page, make changes, take a look at the preview and then cancel your edits. No one will be the wiser that you played around with a page.
What has changed?
Now that you have made a change, one thing that you should know is that all changes to a page are tracked and are visible to everyone and attibutable to your account. This provides some accountablity and the ability to see exactly what has changed between various page revisions. The best way to see what has changed is to select the recent changes on the page. This will show you the most recent changes to all of the pages in the wiki. From this page you can see who made the change, when it was made and what was changed.
If you know what an RSS feed is, you can get an RSS feed of the recent changes page which will allow you to easily keep up-to-date with all of the site changes using an News aggregator (like bloglines, Feedreader, Mozilla Thunderbird, or on found in this list of news aggregators).
If you would like to discuss something that is present on a page, use the discussion link that is present near the top of the page (for the default skin). This will allow you to ask questions or make comments pertaining to the page that has the discussion link. Anyone can answer or provide additional comments.
If you are still lost and cannot figure out how to use this wiki to make changes, send me a question and I will try to help you out.