Hi there.
There are a few open RPs that are accepting people, but the expectations of writing can be a bit advanced for some of them. Some are more free-form and forum-based, but most actually do the RPing over instant messaging and then are summarized and put in their threads.
Take a look at the front page of our section and pay attention to which threads have RP, Role-play, or community project in the titles. If any of the active ones interest you, send a private message to the one who posted the thread, since they are usually the ones who run them, and ask them about joining. If it doesn't work out, you're also welcome to write up your own and run it by the regulars here. You might attract a few people if your world or system is unique enough.
Best of luck,
-X