Just to be clear, all of your work is your copyright as soon as you have written it. There is no magic task you have to do to get copyright. You already have the copyright.
However, proving that you have copyright is another matter. This is where registration is so useful. The question is, what would a court accept as proof that you own the copyright?
Oh, also, if someone uses your registered copyrighted work without permission, and they get caught, they are required to pay you back royalties from the time they began using your work, as well as any future uses.
If you catch someone using your non-registered copyright work, you can only collect royalties from future use of the work after they have been notified by you of their plagiarism.