UPDATE for foreign travelers taking a tour to the US.
All Visa Waiver Program (VWP) Travelers to the United States must obtain authorization via the Electronic System for Travel Authorization (ESTA) prior to traveling to the United States. Find out if your country participates in the Visa Waiver Program (VWP): https://travel.state.gov/content/travel/en/us-visas/tourism-visit/visa-waiver-program.html
Apply for the ESTA here: https://esta.cbp.dhs.gov/
For travelers of countries not included in the Visa Waiver Program, a valid U.S. Visa is necessary for entry into the United States. Apply for a U.S. Visa: https://www.ustraveldocs.com/
For any other questions or clarifications, you may call the US Customs and Border Protection Information center at 202-325-8000.