To enter Mexico you need to obtain a Tourist Card (FMT). To get your Tourist Card you will need to prove citizenship, a passport is best for this purpose. Other forms of accepted proof of citizenship include a certified birth certificate, voter's registration card, naturalization papers, or a notarized affidavit of citizenship. A passport can serve as proof of citizenship and photo ID. Airlines will furnish the Tourist Card on board. Be sure to save your copy, as you must present this when leaving Mexico.