Before you can claim your patronus, you need to first explain why you believe that animal should be yours. From the book, we know that the patronus animal conveys something about that person, so how would your patronus represent you? Nothing too long, but long enough to convince myself or my co-moderator acciofirebolt that you deserve that patronus.
Also, just a few rules that shouldn't be too new to anyone. 1) Of course only one patronus per person 2) Be respectful if someone has already claimed the patronus you were hoping for. Everyone is different, so I'm sure you will be able to think of another animal that could equally represent you. 3) No fighting. Period 4) This is just for fun for us crazy HP fans, so just keep that in mind!