Passport, or proof of citizenship (such as original birth certificate or naturalization certificate) and photo ID.  Tourist card is required.  Tourist card valid 3 months for single entry up to 180 days, $20 fee, requires proof of U.S. citizenship, photo ID, and proof of sufficient funds.  Visa not required of U.S. citizens for tourist/transit stay of up to 30 days. 

Obtain tourist cards in advance from Consulate, Tourism Office, and most airlines serving Mexico upon arrival.  Departure tax $10 is paid at airport when not included in the cost of the airline ticket.  Notarized consent from parent(s) required for children under age 18 traveling alone, with one parent, or in someone else’s custody. 

For additional information, check with the Embassy of Mexico, 1911 Pennsylvania Ave., NW, Washington, DC 20006 (202/736-1000) or nearest Consulate General: AZ (602/242-7398), CA (213/351-6800, 415/392-5554 and 619/231-8414), CO (303/331-1110), FL (305/716-4977), GA (404/266-1913), IL (312/855-1380), LA (504/522-3596), NY (212/217-6400), PR (809/764-0258) or TX (210/227-1085, 214/630-7341, 713/542-2300, 512/478-9031 and 915/533-4082). 

More information:
http://www.embassyofmexico.org/