If you wish to travel to the US from a Visa Waiver country, you need to to apply for an ESTA (Electronic System for Travel Authorization) before you travel.
The Electronic System for Travel Authorization (ESTA) is a United States government requirement (mandated by the Implementing Recommendations of the 9/11 Commission Act of 2007) for participating travelers from Visa Waiver Program (VWP) countries. The ESTA is available for travelers from participating countries arriving by air or sea, to complete online the I-94W form (formerly paper-based and filled on arrival by participating VWP citizens) in advance of travel to the United States. ESTA is not needed when arriving by land from Canada or Mexico.1 Passengers (including babies) without an ESTA may be denied check in, although it is possible to get one while at the airport. Since 2010 the payment of US$14 is required to obtain the ESTA.