This is only partially written, but for now, much of the information on MediaWiki.org can be used (and will be linked below). What differs on CassWiki is mainly the use of editing forms. Apart from the big text field, when editing pages there are also other areas to be found on the "Edit" page for entering information.
Getting to the create/edit form
In the sidebar to the left, you'll find a link to create or edit a page. That page lists forms used to create or edit pages. In most cases, the "General topic" form is currently the one used. If you follow the link for a form, you can type in the name of the article you want to create or edit and proceed.
If the page already exists, the edit screen will say "Edit" and you'll see the existing contents within the form. If the page doesn't exist yet, the edit screen will say "Create", and after you fill out the form and click "Save page" at the very bottom, the page will be created.
If a page already exists, then another way to get to its editing form is to go to the article and click the "Edit" link above it to the right. However, this won't work with "Create" for creating new pages by first going to them, since then the form is not yet associated with the page – and you'll end up at the "standard" editing interface, which should be avoided unless you know what you're doing (manually handling special wiki code common to series of pages).
Using the create/edit form
At the top of the edit form, there's fields for entering information about the page, which is then used to list it in various places automatically on the wiki. Next to each of those fields, there is a small info icon which, if hovered over, provides more information about that field.
Below that, there's tabs on the edit screen, within which the contents of the page are entered. There is the "Article text" tab, where wiki text is entered in the usual way, as described in the links below. There is also the "Resource links" tab, where links to external resources can be entered in a (hopefully) user-friendly way, specifying title, URL, description, etc. Headings and lists are automatically shown below the text for any resource links added to a page.
Beyond that, the 'Editing pages' page from MediaWiki.org applies.
When editing, in the large text entry field (used for the main text of the article), you'll see a "Help" link in the toolbar. Clicking on it, a quick reference will drop down below the toolbar. It summarizes the information most frequently needed for writing content. More extensive information can be found in the links below.
Here's some wiki-formatting help from MediaWiki.org: