Your Works are Automatically Protected by U.S. Copyright Laws
Under U.S. law, you have certain rights (copyrights) to anything you create that falls into the category of "forms of expressions" covered by copyright laws.
Since many works are not covered under this "automatic" law, be sure you understand the differences between copyrights, patents, and trademarks.
For this automatic protection to exist you do not have to register with the U.S. Copyright Office, or even have published your work. For unpublished works, however, there must be some form of tangible proof of when you created the “expression” or material, and that it is your creation.
Automatic Protection is Not Fool-Proof
But even with an automatic copyright, anyone can contest your rights (or you may need to assert your own rights), so only you can decide whether or not something you created is worthy of formal copyright registration.
Internet plagiarism is very common. It is easy for someone to say they created something, when in fact they did not. Sometimes copyright infringement is truly an act of ignorance, but copyright infringers can still be held liable if they take credit for things they did not create.
Formal copyright registration serves as more substantial proof that you are the creator (author) of something, and when you created it. Simply putting “Copyright 2008” on a website does not really prove that you actually created the material, and if so, when.