For travel to the States from the UK you now need to apply for your visa online. There are a number of websites that offer you the service of checking your visa app for you but don’t be tempted to spend more than you need to as the process is very simple. Go to https://esta.cbp.dhs.gov/esta/. It costs $14, the visa lasts for 2 years. Before you begin this application, make sure that you have a valid passport and credit card available. This application will only accept the following credit cards: MasterCard, VISA, American Express, and Discover.
Usually you get your answer on line straight away, but you should allow 72 hours before you fly to ensure you get your visa before you fly. You don’t need to take a copy of it with you to the airport although a lot of passengers do find comfort in having it with them!
As it is valid for 2 years, you can also go back into your application and update the information if you travel again to a different address or with a different airline or from a different departure airport.