First, I’d like to clear up a common misconception. You cannot copyright ideas. You can, however, copyright the sequence of words that convey those ideas.
Using a copyright symbol on unpublished manuscripts that you’re sending out for consideration at literary journals, publishing houses, or agencies is unnecessary. In fact, including it on your manuscript has become the mark of an amateur. Your work is copyrighted the moment you write it. The simple fact that the words are committed to the page already lets any reader know that it is copyrighted. Including the symbol offers no additional protection.
Some writers file their work with the Library of Congress’ Copyright Office, which offers proof of the date of filing. This could come in handy if you were ever called by a court of law to prove you wrote the work earlier than someone else claiming to have written it. It also allows you to sue for infringement.
However, the theft of unpublished work is incredibly rare. This is particularly true if you send your work to publications and organization that you know to be reputable. Still, even people out to con writers aren’t going to steal manuscripts. There’s not a lot to gain from it. They’re more likely to set up a fake front as an agency or editor and charge high fees for work they don’t do.
Once your work is published and reaches a much wider audience, you will want the benefits that come with copyright registration. And that is often done as part of the process of publishing the work.