The page has been created, you can go ahead and edit it.
Start by simply entering your text in this edit box. The icons above the edit box will help with the most common markup tasks (bold, lists, links, etc.), plus there is a wiki syntax page with more details about markup.
Before saving your changes it is a good idea to use the Preview button. Unfortunately editing is not WYSIWYG.
Go ahead and delete all this text except the top line (the header) and add your content!