At the bottom of every page is a link saying "Edit this page". Clicking that will allow you to try it. Go to SandBox (a playground junk page) and try it real fast. Just type anything in the field. This page is locked, to prevent it getting stomped on accidentally. You can make links to other people's wiki pages by typing WordsJammedTogether?, aka WikiWords?. It's the alternating high and low caps that make it a link. Alternatively, you can put a non WikiWord? in [[double square-brackets]]. That way you can make links out of words like Discussions? or your name, if it can't be typed as a WikiWord? (like Andres?).
To go to a page, you can just enter it into the URL for the site after the slash. For example, this page can be found at http://fading-suns.mapache.org/QuickStart If the page does not yet exist, you have the opportunity to create it by editing it. Pages can have one level of nesting. This is, http://fading-suns.mapache.org/QuickStart/NestedPage Nested pages automatically have links back to their parents, and you can link from the parent to the nested page by typing just its name preceded by a slash, like /NestedPage. They can also be accessed from anywhere in the site via the whole name, as QuickStart/NestedPage.
Look around the site and get a feel for what people are doing before you edit.
You can format text as you edit. Check out TextFormatting.