Okay, the most important thing you'll need to do is start creating/obtaining content. You can either write your own pastas & post them, or find them on the internet & post them. A good place to find pastas is Reddit's NoSleep board, be sure to give proper credit (a link to the pasta on Reddit & the name of its author). If you write many of your own pastas, you might want to start creating advice for others, so they have a reason to use your site.
You'll want to get a wordmark for your wiki - that's the logo that appears in place of the text in the top left. Either make it yourself in photoshop/GIMP or find someone else to make it for you - it must be 250x65px and uploaded to File:Wiki-wordmark.png. Otherwise your wiki just looks unprofessional. Additionally, you might want to set the background image to be fixed, as it currently just disappears at the bottom of the page.
It might be a good idea to establish some clear rules and guidelines. Also, you might want to enable chat. We disabled it at the original Creepypasta Wiki because of excessive drama, but it's a good way to attract initial contributors.
Lastly, don't be afraid to use the many special pages you have available. They can be extremely useful.