As an author, it is important that you protect your creativeness by knowing what is available to you under the U.S. law.
I'll start by saying this is in no way legal advice and I am not a lawyer; this is merely something I've come across in my own writing journey that is good information to pass along to my fellow writers.
What a Copyright Isn't
You cannot copyright an idea, nor is it possible to copyright a concept or how to do a certain task. Even a title cannot be granted a copyright. For instance, go to Amazon and type the word "hunt" into the search. You'll find more than 20 search result pages of books and movies called Hunt (or some offshoot of the word). Under the law, this is completely fine and legal, and truthfully it's not all that uncommon to find the same title used over and over again.
It's also not uncommon to find the same idea used over and over again. Case in point, if you compare the movie Dances with Wolves and Avatar, you'll see very similar threads of story. Same goes for Harry Potter and Star Wars.
It's important to note for my fellow authors that if you have someone threaten to sue you over a title, don't let it cause any heartburn. No court would listen to them.
What a Copyright Is
A copyright is a public notice of the claim to your own work. It protects your specific story from plagiarism and provides more protection under the law should anyone decide to sue you over it. Your story is yours.
This also provides protection in the event you as an author need to file suit against someone else if they copy your work. It allows you to claim financial damages in a court of law. In fact, if a copyright is filed within the first five years of the publish date, it is considered prima facie evidence in a court of law.
Authors are not required to file a copyright. Everything written by anyone automatically falls into what's known as common copyright. Formally filing a copyright is required, though, in the event an author needs to bring suit in a court of law.
Filing Copyright
Filling with the U.S. Copyright Office is fairly simple and straightforward. There is a fee involved, registering with personal information as a user, and sending two of the "best versions" of the book to the Copyright Office (this is also known as mandatory deposits and is how the Library of Congress obtains books).
More information on filing a copyright can be found at https://www.copyright.gov.