To edit a wiki page, click on the "Edit this page" (or just "edit") link at one of its edges. This will bring you to the edit page: a page with a text box containing the wikitext: the editable source code from which the server produces the webpage. Depending on the preferences that has been set, a version of the edit page called preview page appears: in this case the rendered page is also shown; if not you get it by pressing "Show preview", see below.
To create a new page, either follow a red link in an article (red means the link has no page yet), or type http://www.arrse.co.uk/wiki/index.php/put_the_name_of_your_page_here in to the address bar on your browser. If it doesn't exist, the software will open a blank page for you and you can type away!
Please add your page to an existing category as explaned in the link below, although if you can't work it out, don't worry too much.
Further details can be found on the MediaWiki Edit page?