And as a practical matter, most states do not attempt to assert copyright over anything except perhaps computer software or other high-cost items. If you are writing the great American novel on your word processor, then the novel is being copyrighted as you enter it into your computer. Hence you would not be creating a copyrighted work in that circumstance. You may, if you like, register a copyright with the Copyright Office. But registering a copyright is not the same thing as having a copyright.
Registration is handy for a few reasons, though far from essential.
If you think you might ever sue anyone for copyright infringement, registering early offers some advantages in the amount of damages you might be able to collect. At one time, you were required to put such a notice on any work you wanted to protect whenever you published the work. Others who saw a published work without such a notice were entitled to treat the work as being in the public domain. In other words, our law carried a presumption that a published work was not copyrighted unless it carried the copyright notice. Since 1989, however, our laws no longer require a copyright notice.