Any one can read the pages in the Wiki but in order to edit pages you will need to request permission of the Wiki administrator to become a Writer.
As a writer when you open a page you will see (top left) an Edit tab. Click this and the editing screen opens. Most of the time you will just want to retype over what is there, or add paragraphs. When you have finished click on Save (bottom left) and review your work.
In the editor you can use the toolbars to format text (bold, italics, etc), headings (1,2 &3) and so on. Pictures can be inserted and resized easily too. Links to other pages in the Wiki and to external sites can also be inserted.