It's probably safe to say that most people want to be creative. It's also probably safe to say that a lot of people don't think they have the time to be creative.
This community says they're wrong.
Format doesn't matter; writing, drawing, music, whatever. Length doesn't matter; you could toss out two hundred words on potatoes or the ceiling of the Sistine Chapel. It doesn't even matter that it's original to this journal - so long as you've created it in the last week. All that matters is that you post one a week. That's it. You don't have any other commitments to the community.
Some posting guidelines:
Writing: You don't have to post a word-count with what you write, or how long it took you to write it, but it's encouraged. (Some people, the moderator included, think it's neat to know how long it took someone to write a given piece.) If what you're posting is more than about a thousand words, please use <lj-cut> to hide it, to save members' sanity when they're reading their Friends lists. Likewise, if it has any adult content at all - swearing, graphic violence, sex, et cetera - please cut it away; this is more to make the community work-safe than anything else. Otherwise, the only limits on posts are the hard-coded length limit on the post itself. (It is okay to link to a story hosted on a remote server, if the story is too long to fit in a post!)
Graphic art: You don't have to post what materials you used or how long it took you to create the piece, but it's encouraged. (Again, some people think it's neat to know that kind of thing.) The same sorts of limits apply to art as to writing: if the image is larger than about 400x300, or if it contains adult content, please cut it away. Again, it's okay to link to a remote server.
Music and Sound: Once more, you don't have to post what instruments you used or how long it took you to create the piece, but it's encouraged. Please do not embed music files into the HTML! (Embedded music drives the moderator nuts.) Also, please note in the post whether the music contains adult content.
Other: The same basic rules apply. Cut the post away if it has adult content, and use your discretion (and use the examples in Writing and Graphic Art above as reference points) when posting long works.
Keep in mind that if you don't follow the guidelines above, you run the risk of the moderator modifying your post so that it does follow them.
Keep in mind also that anything you post must be your creation. Plagiarism in any form is strictly disallowed. (Fan-fiction, fan-art, and the like, on the other hand, aren't disallowed. Original works are preferred, but if you have a great Saiyuki/Neon Genesis Evangelion crossover that you want to post, go for it.)
A note regarding comments: You aren't required to comment on all of the posts in this community. You aren't even required to comment on any of the posts in this community. Part of the point of the community, though, is to allow its members to get feedback on their work, and although it's a somewhat regrettable social construct, the fact remains that the fewer comments you leave, the fewer comments you'll tend to get.
Also remember: if you don't get any comments, that doesn't mean anything. Don't assume that people think that your post is horrible (or that you're horrible) if they haven't left comments. It's been the moderator's experience that if people think something is horrible, they'll say so.
On the other hand, if you are inclined to comment, please take the time to be specific. Comments on creative work should be helpful, and "this sucks" is a lot less helpful than "this sucks because of x". Likewise, keep in mind that in critiques, words like "should" and "must" are discouraged; try "you might want to do x" or "I think x works better".
Other than that, there aren't really any rules. The community does require discipline - but in the end, this is supposed to be fun.