Have fun, but be serious
If you've edited Wikipedia, don't worry. MTS2Wiki won't be as stuffy and rule-ridden as that. You can have some fun here, but remember, we're trying to build a usable history and database for the community.
Use the Talk pages
At the top of each article (or template, or image) is a "discussion" link. This'll take you to a Talk page for the article you're in. It's a good spot to discuss potentially controversial stuff you plan on doing, ask questions, etc. Remember to sign your comments with four tildes (~~~~). To start a new discussion on a talk page, click the + link at the top of the page.
Look at what others have done
No need to reinvent the wheel, right? Chances are, when you start editing, there are already some pages similar to what you want to wind up with. Use them to your advantage! Want to know how someone did that spiffy infobox? Click the "edit" link at the top of the page and look at the code. Hell, copy it wholesale.
This is a good way to learn Wikiformatting, too. Using proper bulleted and numbered lists, Wikilinks, section headers, etc. will go a long way to making the Sims2Wiki easily readable.
You can figure out what categories to apply to your articles this way, too. You're free to invent your own if you've ideas on how to improve the organization around here, of course!
To create a page you can either use the Search box to search for the term, and then click the resulting "Create this page" link that shows up, or you can type the URL in directly, for example http://sims2wiki.info/DummyTestPage
Once you have composed your new page to your satisfaction, use the MOVE tab, (found above the edit box), to give it the correct title and location.
Chances are someone is going to be an ass and disrupt the Wiki. You can always reverse their changes - unless they're an administrator, they can't actually delete anything from the site permanently. If someone gets way out of hand, we're able to ban them if it comes to that.
That said, do try and resolve disputes in a polite, happy way. Think of how you'd act on the Forums. Pretend the moderators are breathing down your neck. Fighting over an article? Take a step back, discuss it in the Talk page, and maybe ask some others to come give their opinions on the matter. We're all here to improve the Wiki, right?
The best reference on the Wiki formatting is available on WikiMedia, here: 
Most of all, enjoy your time on the Wiki. This can be a fun project for all of us - no more "hey, who the hell was <so and so>?"