Citizens of all countries except Canada are required to have the appropriate visa in their passports if they are to enter the U.S. in a specific nonimmigrant classification - be it to study, to work or conduct research, or even to be a tourist. In certain limited circumstances, citizens of some countries are exempt from have a visitors visa when coming to the U.s. as a visitor for business or for pleasure for periods of 90 days or less.
Visas can only be obtained at a U.S. consulate outside the United States. If you have a valid visa in your passport for the immigration status you are seeking, you generally will not need to obtain a new one. If the visa is expired or if you changed your immigration status inside the U.S., you will need to apply for a new visa.
Application procedures vary considerably from one country to another. Most consulates interview prospective students. If you have a visa interview, it will be short. During the interview, you must make a good impression. Be positive and respond to the questions with clear, concise answers. If your spouse and children will remain in your country, be prepared to explain how they will support themselves in your absence. For the most up to date information, contact the appropriate consulate.
To learn how to apply for a visa, visit www.travel.state.gov.
Learn about the https://j1visa.state.gov/programs/professor.