Copyright in the U.S. is immediate upon creation of a work.
GP is partially incorrect; you can bring a suit regarding an unregistered work. In such cases, however, you cannot claim statutory damages, but are limited to claiming actual damages (actual money lost or actual profit from the infringer). If the work is registered within the alotted time, you can claim statutory damages and attorney's fees.
You can register within 1 month of learning of the infringement to protect your ability to claim statutory damages and attorney's fees.
GP is partially incorrect; you can bring a suit regarding an unregistered work. In such cases, however, you cannot claim statutory damages, but are limited to claiming actual damages (actual money lost or actual profit from the infringer). If the work is registered within the alotted time, you can claim statutory damages and attorney's fees.
You can register within 1 month of learning of the infringement to protect your ability to claim statutory damages and attorney's fees.
https://www.law.cornell.edu/uscode/text/17/412