The basic idea of this web site is that anyone can edit any page. Just follow the EditText link at the bottom of a page to edit it. A special text format allows you to create pages simply, without using HTML. Any text consisting of at least two capitalized words with the rest of their letters lowercase is turned into a WikiWord and automatically linked to a page with that name. You can follow such links to nonexistent pages to create them. To create a new page that is not yet linked, edit another page to link to it, and then follow the link from the updated page.
The most important thing to keep in mind is that backups of all pages are saved automatically. You can see the old versions of a page by clicking on the 
button at the top right of it and choosing "Show 'Revision History'". This means that, as long as you want to edit or add to a page in good faith, you should never be afraid to do it. If you were to "mess things up," which is probably unlikely, any of us can clean up your changes easily. Usual "netiquette" about not playing around with "other people's web sites" does not apply here. This web site is maintained by anyone who wants to put time into it, possibly including you. Just spend a small amount of time experimenting by editing the WikiSandBox, and then you should be ready to contribute.
For convenience in keeping track of who edits what and in storing some customized preferences that you might want to set, this Wiki supports simple user accounts. They are actually stored only in your local browser as cookies, and they are not intended to provide any additional "security," just convenience. Follow the UserPreferences link at the top right of any page to create such an account. The WikiWord name you choose for yourself will be used in the history records of pages you edit.
If you do create an account and use it regularly, you should edit the page with the same name as your account, basing it off of the HomepageTemplate. The template contains a space for your e-mail address formatted with a special macro. While your e-mail address will appear in a normal mailto link if you follow this format, you don't need to worry about web spiders harvesting your address to spam you. The mailto link is only shown to people who are logged in, while anyone else will see an obfuscated version with no link.
Once you're all set up with an account, the easiest way to get into the thick of things is to bookmark/visit first/etc. the RecentChanges page instead of FrontPage. This page tells you which pages have been changed, when, and by whom, providing a centralized and simple way to know what's happened since you last visited. When logged in, you can save a bookmark to record when you last finished reading the list of changed pages. Veteran wiki users are notorious for reloading RecentChanges very often.
For further information, try HelpForBeginners or the DefaultFrontPage for this site software.