Welcome to the Sandbox
Here is where you build some Wiki Skills.
At the top of the browser window you'll see a little tab that says "edit". Go ahead and click it. In a moment the screen will refresh and you will be in the editing window.
Type some text into the pane, and click the button, below, that says "Show preview" and it will show you a preview that you can either save, or edit some more. When done, press the "Save page" button below.
That's all there is to is, unless you want to get fancy.
To learn how to get fancy, please visit Mediawiki's Help On Editing page.
To get back to the Main Page, click the link in the "navigation" pane, to the left.