First of all, ideally you will read the UsingWiki document first, and then read the CheatSheet for more details. But, for quick and dirty:
1) Click Edit This Page at the Bottom of the page to be edited.
2) Put your real name in the Username box on the edit page.
3) Make your changes to the text, mimicking surrounding formatting.
4) Click save. Put in the password and username (human, human) suggested by the prompt (this keeps out computerized spammers)
Done!