To obtain an electronic authorisation simply log onto the ESTA Web site at https://esta.cbp.dhs.gov and complete an on-line application form. Applications must be submitted prior to travel and the US Department of Homeland Security recommends that applications be submitted no less than 72 hours prior to travel. However, please note: Any travelers who are not authorised to travel to the USA using this system must obtain a nonimmigrant visa at a U.S. Embassy or Consulate before traveling to the USA and this should be done as soon as possible as applying for a US visa is/can be a very lengthy process.
All passengers entering the USA on the Visa Waiver Programme will be required to hold a machine readable passport (the newer red passports). This also includes passengers who only intend to transit the USA en route to another country.
What is a Machine Readable Passport - A MRP has the holder's personal details, e.g. name, date of birth, nationality and their passport number contained in two lines of text at the base of the photo page. A machine can read this text.
The UK have been issuing MRP's since 1988, however British passports issued overseas may not be machine readable.
For the latest information and advise on international travel please visit www.fco.gov.uk