Registration and renewal is not required (although it used to be).
A work is copyrighted as soon as it's entered into a "tangible medium".
Scrawled on paper, etched in stone or saved to (written on) a magnetic disk, for instance. (Copyright only applies to works "fixed in a tangible form.")
Strumming out the chords to "Pink Moon" doesn't get you copyright in it until you've recorded it or written it down.
It doesn't have to be marked as copyrighted. Copyright notices are only helpful in that they let you know who the copyright owner is so you can contact them. Not required.
You should generally assume that all works are copyrighted.