Check with the Mexican embassy or consulate to see if you need one. You may not need anything more than your passport.
Most countries not considered to be "developed". Some examples include: Brazil China India Russian Federation Nigeria Peru Vietnam
There are several different ways to obtain a visa to go to India. There are many websites online, too. Visa Travel is a website that offers everything needed for a U.S. citizen to obtain a visa to...
probably so. Seek one, just to be safe, or go to the nearest embassy
simply H E L L
It depends on your country of origin but in general terms if you belong to a developed country you don't need anything besides your passport.