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You need to apply for travel authorisation via ESTA (Electronic System for Travel Authorization). This is a straightforward process that takes a few minutes, and costs $14. Once you have applied, it is valid for 2 years, for multiple entries.
All you need is a valid passport. However this passport must meet the following criteria:
The passport must have a machine-readable zone on the biographic page.
Issued on or after October 26, 2005 – Each Visa Waiver Program passport issued on or after this date must have a digital photo.
Issued on or after October 26, 2006 – Each Visa Waiver Program passport issued on or after this date must be an electronic passport with a digital chip containing biometric information about the passport owner.
It will also ask you for the address you will be staying at during your trip, as well as the flight or vessel number you will be arriving on. If you haven't booked travel yet, you may be able to leave these blank, or just guess what they are likely to be.