The first answer is correct that you apply at the US Embassy. However, your best first step is to make contact with a travel agency that is familiar with the documentation requirements.Good luck!
You will have to apply to the US Embassy in London in person.US Embassy,24, Grosvenor Square,LondonW1A 1AEThis will give you all the details.http://www.usembassy.org.uk/cons_new/vis��The Visa Department is by Appointment onlyhttp://www.usembassy.org.uk/cons_new/vis��