You will need your original birth certificate or a valid passport along with a valid photo ID like a drivers license to enter Mexico. If you are not an American or Canadian citizen, you may have other requirements like a visa. Contact a Mexican embassy or consulate near you.
You are required to have a Tourist Card if you are coming to the country by any means other than cruise ship or airlines. You will also need it if you are staying for more than 72 hours or traveling over 20 miles beyond the border.
If you are entering Mexico by cruise or plane, you will need to present your identification at the cruise/airline check-in. Once on the ship/plane you will have to fill out two forms – one for Customs (Customs Declaration Form) and another for Immigration (Tourist Card).