All the information you need to know about visiting a specific country is available from the U.S. Department of State at the link below, organized by country.  Information on entry requirements is in the Entry/Exit Requirements section.  Other useful information includes the locations of the U.S. embassy and consulates, unusual immigration practices, health conditions, minor political disturbances, unusual currency and entry regulations, crime and security information and drug penalties.

For more information, visit the Travel State website