Everything you make has a copyright from the instant it was created, such as music, art, and poetry.
Registration is not required. However, if somebody infringes your work (steal) you'll be limited to using state law, not federal law. For a one-time $45 registration fee with the U.S. Copyright Office, you'll have the benefit of federal laws to protect against infringement.
The $45 fee is for basic applications that are defined as "a single author, same claimant, one work, and not for hire." Otherwise, the fee is $65. Other registrations may have a higher fee, such as a "database that predominantly consists of photographs and updates", which is $250. These fees are for online submissions. A paper application has different fees.
Copyright works like a time-stamp. Submit a copy to the Copyright Office and get a certificate with a date when received.
If an infringer claims your work as their own: (1) prove the two works are the same and (2) that you have the earlier time stamp.
The original time-stamp only protects what was submitted the first time. Any revisions to the original copyright require a new registration.
A copyright application requires a copy of the best edition of your original work to be deposited with the U.S. Library of Congress.
What is a Copyright?
The owner of a registered work has exclusive rights.