To be able to edit articles, you need to be logged in, which in turn requires you to have registered an account. If you don't have an account, you can register on the same page as you do log-ins.
Editing a page
To edit a page, the short version is to click "Edit this page" at the bottom of the article or the "Edit"-link associated with the section you wish to change, change or add whatever you wish to change or add, then click "Save page" at the bottom.
The longer version is located at MetaWikiPedia:MediaWiki_User's_Guide, with most of it in MetaWikiPedia:MediaWiki_User's_Guide:_Editing_overview
Note that the Wiki uses special syntax to make headers, links, emphasis, insert images, etc, etc. You'll find a complete overview of it in the MediaWiki's User Guide, but if you simply wish to see how something on a given page was done, you can click "edit" and look at the syntax used (clicking "edit" doesn't affect the page you are looking at).
If you wish to experiment with how things look and work, feel free to do so in the Sandbox.
Creating a page
To create a new page, first you need to make a link to it, e.g. by entering "[[Title of new page]]" on a page you wish to link from. Then save the page, and click the link (which will be in red as long as the new page doesn't exist). You'll be taken to the new page's editing screen, and from there it's just like editing an already existing page.
We haven't set up a long list of rules for what you can and cannot do; what is can be found at VickyWiki:About. Since everyone registered can edit everything, everyone can act as censors as well.
Rule of thumb, anything you could've posted on the Paradox boards, you can put here as well, just as long as it is relevant to the game.
Any content deemed illegal or inapproriate will be removed. The sysops can and will ban users who disregard the spirit in which this wiki has been set up.
Note that the wiki is not a discussion forum; the official Paradox forums are way better suited for that. VickyWiki is meant as a reference-site and an encyclopedia.
If you are considering posting something that you aren't sure about, feel free to ask the sysops.