Unless you have ESTA authorisation to travel or a visa, you'll be denied entry into the country.
ESTA stands for Electronic System for Travel Authorisation. It allows you to travel to America without a visa if you’re travelling from a country that's part of the Visa Waiver Program.
The Visa Waiver Program is a list of 38 countries whose citizens can travel to the States without a visa, as long as they have travel authorisation. The United Kingdom is included in the list of 38. As long as you qualify for an ESTA, you won’t need a visitor’s visa.
You'll need to apply for your ESTA before you travel to the US by air or sea.
If you hold a British passport with an electronic chip in it and you plan to visit the US for business or pleasure for less than 90 days, you can apply for an ESTA. Even if you're only transiting through the US, you’ll still need an ESTA. If you're going to the States for longer, or plan to live and work in the US, you should apply for the relevant visa.
You'll need to apply for your ESTA before you travel to the US by air or sea. If you come overland, from Canada for example, you'll be asked to fill in a different form, unless you already have a valid ESTA.
Remember to put your worldwide travel insurance in place before you travel as well. Medical treatment in the States is expensive. There’s no equivalent to the NHS and no special arrangements are in place with the US, like there are with many European countries.