First of all, European and Australian travellers must have an ESTA to go on a USA road trip.
Of course, you also need a biometric or electronic passport to cross the American border.
Your trip must be for 90 days or less, and you must show a round-trip airline ticket as well as proof of solvency (credit card, traveller's cheques, etc.).
Visit our "Passport and Formalities" section to find out more.