To cross the border, Americans and Canadians need:
1. A passport with a six month minimum validity
2. $500 in cash or traveler's checks (to demonstrate financial solvency)
3. A tourism card (tarjeta de tourismo)
4. Some kind of ticket – bus, plane, etc. out of the country. If you are with a tour group this is usually not necessary.

British citizens do not need the tourism card, but the other requirements remain the same.
To get the tourism card, first go to one of the immigration windows and present your other documents. Then you will be given a slip of paper that says “tarjeta.” Take this to the tourism office located behind you, where you will be issued an authorized card. The card costs $5.
The migration office is open from 7a.m. to 11p.m. daily.