Hello! Thanks for your question.
You can't copyright ideas, only the way they are expressed. Ideas, concepts, systems etc. must be compiled into a "work" and be fixed in tangible form such as a CD, audio file, script, book, etc. If you want specifics, download the Copyright Basics
pdf from the United States Copyright Office.
On the event you develop your idea into a script, then you definitely want to register your work with the US Copyright Office. Although technically, when you produce a work it is legally copyrighted, enforcing that (and proving it) is another matter. That is why you register your work with The US Copyright Office.
It is standard for writers to register their works with WGA, however this is NOT a copyright. The only thing WGA does is document the date you registered your work. If you had to go into court and try to enforce your work, the only thing WGA would do is verify the date of registration with them. Copyright is always the way to go in addition to registering with WGA.