If you are not a US Citizen or Permanent Resident, you will need a student visa for the duration of your studies at Northeastern. The application process for the student visa requires the submission of documents to Northeastern and an appointment at your local US Embassy. This process usually occurs sometime in May, June or July.
If your internship is outside of your home country and you will not return to your home country before departing for Boston, you will need to book an appointment at the US Embassy in your current location. The Embassy will likely require documentation that proves that you are in the country of your internship legally (i.e. proof of a visa and/or letter from the employer stating employment dates). Please be sure to contact the US Embassy responsible for the location of your internship to inquire about specific documentation required.
If your internship is in the USA, you will not be able to transfer the internship visa to a student visa. You will need to apply for a new visa for the start of the program in Boston. This will require that you leave the USA and make a visa appointment at a US Embassy. This may require that you return to your home country. There is no guarantee that a US Embassy responsible for a country outside of your home country will allow you to book a visa appointment without specific documentation for the reason that you are in that country. Be sure to consider this when planning your internship an allow yourself a few weeks in your home country before the start of semester 1 in Boston to manage the visa application procedures.