How do i copyright my short film?

Like anything else; you automatically owned the copyright from the moment you first put your creative work of original authorship into any tangible form. In case of a film, that may have occurred when you wrote the screenplay. You may remind people of your ownership by attaching a copyright notice. This makes it easier for people to figure out who owns the copyright, not to mention when it was published or authored, if that matters.

Perhaps you should consider registering your copyright ownership, which is completely optional, but provides you certain additional legal rights, under US copyright laws. Most countries have no possible way to do such a thing.