Generally, if you want to visit (and not live in) the United States you must first obtain a visitor visa. Travelers from certain countries may be exempt from this requirement. For more information, please see the U.S Department of State website.
If you want to travel to the United States for reasons other than business or pleasure, you must apply for a visa in the appropriate category. This includes if you want to study, work as a crew member or journalist, etc., You can get help determining which visa you need by selecting the appropriate categories on our home page.
Extending Your Visit
If Customs and Border Protection (CBP) authorizes your admission to the United States at the designated port of entry, you will receive a stamped Form I-94, Record of Arrival-Departure. If you wish to stay beyond the time indicated on the Form I-94, you may apply for an extension by filing Form I-539, Application to Extend/Change Nonimmigrant Status, with USCIS.
If You Lose Form I-94
You may apply for a replacement Form I-94 by filing a Form I-102, Application for Replacement/Initial Nonimmigrant Arrival/Departure Record.