If you have sufficient access, you'll see "View" and "Edit" tabs at the top of each page. To edit a page, click on the "Edit" tab, make your changes, and save.
Create a link to a Wiki page that doesn't exist yet. Then click on the link, and you'll be able to create the missing page.
Note: to create a new page you can just add the link you want into the address bar of your browser.