United will ask you either during check-in or at the gate for the APIS (Advance Passenger Information System) data. You will need to provide an address in the US you will stay or if you are on a tour the address of the first hotel you stay in or if you don't have a hotel yet but renting a car the address of the rental car station. Just the name of the hotel or rental-car station is not enough. You need the actual street address, town, state and ZIP-Code.

For the I-94W immigration card and the custom declaration card which you will get probably on board the aircraft you need also the address together with things like your name, country of citizenship, country of residence, sex, date of birth, town your flight departed from, flight number, passport number, etc.. For how to fill out the I94-W you should look into the inflight-magazine. Airlines must have a description in their inflight magazines if flying to the US.