To register means to sign up or enroll for something, such as a service or program. To deregister or deactivate means to cancel or terminate a registration or enrollment.
Here are some common phrases that you might use to describe the process of registering or deregistering:
"I need to register for this class."
"Can you help me register for this event?"
"I need to deactivate my account."
"How do I deregister from this service?"