Most non-U.S. citizens who want enter the U.S.A. need a U.S. visa to do so.
A "visa" is the stamp in your passport that is issued to you by a U.S. Consulate. U.S. Consulates are operated by the U.S. Department of State in countries with which the U.S. has formal relations. There is no way to obtain a new visa stamp while physically in the U.S.
The U.S. visa application has several steps to it. To increase the likelihood of the most efficient processing of your application, we encourage you to read the following overview of the visa application process by clicking on each of the steps above.
If you have questions, contact the Visa Office. [visaoffice@fnal.gov].