As an exchange visitor, you’ll need a J-1 visa to enter the United States. New J-1 visas can be acquired only at a US Embassy or Consulate outside the US. It is not possible to renew a J-1 visa by mail or in person once in the US. Every US Embassy has different visa application procedures, with many requiring appointments for visa processing.
Be sure to contact the US Embassy or Consulate overseas where you plan to apply so that you can ask about additional information and if an appointment is required. Plan ahead and allow ample time to secure your visa.
See the State Department's list of required visa application documents. Anyone applying for a J-1 visa must "prove" to the satisfaction of the Consular Officer that he or she does not intend to immigrate to the U.S. It is important to bring supporting documents to provide evidence of your intent to return to your home country such as: proof of family ties, property, a job in your home country, an apartment/house, etc.
The links on the right should help answer questions you may have, from visa fees and denials to wait times and photo requirements.