International travelers who are seeking to travel to the United States under the Visa Waiver Program are required to complete an ESTA form for entry into the US. These are valid for two years and cost $14. These can be applied for online at: https://esta.cbp.dhs.gov/esta/
International travelers who don’t qualify for an ESTA must apply for a visa from their nearest US embassy.
For more information, visit: https://www.gov.uk/foreign-travel-advice/usa/entry-requirements