The Topic is the one linked in the first post above (Books, Literature, and Writing). The Topic is selected randomly each week, and contestants are chosen from Hubbers who have already written in that Topic.
No, the contestants have already been chosen. You have to have already written a Hub in it before the contest begins. Basically, there's an element of luck. You have to have written in the current Topic before the Topic was picked. We wanted to add a layer of randomness to this particular competition to ensure that we are featuring new Hubbers who write on many different subjects.
So there is no way to know what the topic will be, you just write your hubs and if you happen to have wrote about it earlier then your hubs goes on the list? I was confused, I thought it was a weekly competition people enter after you announced the topic.