Question 1: How do I become a writer?
1 - Join the site.
2 - Click the button that says “Apply to be a Writer” (the one outlined in red in the picture below).
3 - You should go to a page that looks something like the one below. Now all you have to do is give us a reason as to why you should be a writer. And it can't be "because I want to". I suggest telling us what you can offer. A day or two later, you should be promoted (if not contact a moderator or the creator).
Question 8: Can I post other people's work?
Unless an artist or author explicitly say you can, then no. This goes for both art (fanart or otherwise) and fiction (fanfiction or otherwise).
If you find a picture on a public site, such as Aarinfantasy or Y!Gallery you still have to ask the
original artist. If the person that posted the image doesn't know who that is, then you'll just have to live without it.
To put work up that hasn't been allowed to be posted is classified as theft and can lead to some serious repercussions (such as this site being closed down for allowingtheft).
If an artist or author says yes to you posting their work, then you
must link back to them in some way and tell all that you didn't draw/write the image/story. If it is an image you are posting, then you must also write the artist's website on the image itself.