The Electronic System for Travel Authorization (ESTA)

International travelers who are seeking to travel to the United States under the Visa Waiver Program (VWP) are now subject to enhanced security requirements and will be required to pay an administrative fee. All eligible travelers who wish to travel to the U.S. under the Visa Waiver Program must apply for authorization and then pay the fee.

Apply if you have your pasport on this website: https://esta.cbp.dhs.gov/esta/
(best 1 month before departure, min 72h before departure)