Contributing to a wiki is simple.
Across each page you'll find tabs with names like Article, Discussion, Edit, History. Clicking these tabs will allows you to edit a page, or view its revision history or discuss existing content.
Creating a new page
Before creating a new page/article do your research. Find out if something similar exists already and decide if your content could be added there. If not, the quickest method is to type your page title into the search box on the left and click Go. This will present you with a create this page link, click this and start typing.
A good page of instructions on how to use/edit this wiki is here How to Edit a Page
A similarly excellent how-to for images Embedding Images
A cribsheet for wiki markup Cheatsheet
Examples of wiki markup Wikitext Examples