British travellers to the US under the Visa Waiver Programme (VWP), which allows most British Citizen passport holders to visit for up to 90 days without a visa,  

must get an authorisation via the Electronic System for Travel Authorisation (ESTA) prior to their journey.   

Visitors should register through the ESTA website at esta.cbp.dhs.gov and are advised to do so at least 72 hours prior to travel. If you do not have an ESTA you will be refused travel to the USA.