If you're travelling on a Visa Waiver form, the regulations say you must be staying in the USA for less than 90 days, travelling for business, pleasure or transit only, have a confirmed return or onward ticket and present a completed Visa Waiver form (I-94W) on arrival in the USA. (You can get this form when at check- in at your UK departure point, or on board the flight).
Important: Some travelers may not be eligible to enter the United States under the Visa Waiver Programme (VWP). These include people who have been arrested, even if the arrest did not result in a criminal conviction, those with criminal records, (the Rehabilitation of Offenders Act does not apply to U.S. visa law), certain serious communicable illnesses, those who have been refused admission into, or have been deported from, the United States, or have previously overstayed on the VWP. Such travelers must apply for special restricted visas. If they attempt to travel without a visa, they may be refused entry into the United States.
For more information, please visit US Embassy Website