Some countries require US citizens to apply for, and be approved for, a visa to enter said country for business or tourism. To obtain a visa, you must complete the visa application and send the application, along with your physical passport, to the embassy or consulate for the country to which you are traveling. Not all countries require US citizens to obtain a visa. You can find country specific information here.
If you are interested in working through a visa processing service, we’ve had positive experiences with VisaHQ.