Per the Disney Parks Blog:
If you are planning to visit Star Wars: Galaxy’s Edge at the Disneyland Resort between May 31 and June 23, 2019, a reservation and theme park admission are required. 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 the 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.
Guests looking to guarantee a reservation to Star Wars: Galaxy’s Edge today can get one by staying at a Disneyland Resort hotel between May 31 and June 23, 2019.