You must login before you can edit anything.
To edit any page switch to the edit mode by clicking on the edit tab on the top of the page, or edit link to the right of a section. Once it edit mode just type in your text. When you are done, click Save page button on the bottom of the page. As simple as that! Please try it in this Test_Page now.
Wiki allows also a fancy formatting (see the Wiki user guide for the details). For the beginners we recommend looking at the following sections of this guide: