If you travel to the US you need to complete an online visa waiver form (ESTA) before you go. They are free at the moment but a charge of $14 per person will be made from September.https://esta.cbp.dhs.gov/esta/esta.html
So if you are travelling within the next 2 years, or even think you might, apply online now at https://esta.cbp.dhs.gov
and save yourself enough for a double skinny chocca latte with sprinkles.
NB Only apply through this official site; other official looking sites charge you anyway for what is currently a free service.