Star Wars: Galaxy's Edge opens on May 31st at Disneyland Resort. But in order to visit the park, you need to make a reservation. There is no additional cost to make a reservation, but reservations are subject to availability.
Specific details on how to make reservations will be available on Disney Parks Blog and Disneyland.com on May 2 at 8 a.m. PT. You must have a Disney Account to register for a reservation so be sure to create one before registration opens at 10 a.m. PT.