Firstly, you will need a valid passport with at least six months validity from the date of entry into Mexico. U.S. citizens need not obtain a visa before travelling to Mexico if their stay is for 180 days or less. However, if you wish to stay for longer than 180 days, you must obtain the appropriate visa in advance of your arrival.
You will also need to fill out a Mexico tourist card (Forma Migratoria Múltiple or FMM) which can be obtained through the airline or at the point of entry into Mexico. The cost of the FMM is usually included in the plane ticket, but do check with your airline beforehand.
It is essential to check the COVID-19 restrictions and protocols in place by the Mexican government for travellers, as Mexico, like many other countries, has implemented restrictions on entry due to the pandemic. It could include presenting a negative COVID-19 test result obtained within three days of travel and taking a health screening upon arrival, among other measures.
Additionally, some airlines may have their specific requirements, such as wearing face masks and filling out health declaration forms.
To fly to Mexico from US, you will need a valid passport, a tourist card, and adherence to COVID-19 protocols. It is always advisable to check the requirements beforehand with the relevant authorities and your airline to avoid any inconvenience.