Depending on your country of citizenship, your US immigration status, and your flight (in-bound from a foreign country), you may need a US visa to enter the country. Please see https://travel.state.gov/content/travel/en/us-visas/tourism-visit/visitor.html for details.
Visas are not required for eligible citizens of countries participating in the US State Department Visa Waiver Program, please visit https://travel.state.gov/content/travel/en/us-visas/tourism-visit/visa-waiver-program.html
In case you need a letter of invitation for your visa process, please download it from the PaperCept website.
Step 1: Login to the system and find your accepted paper page.
Step 2: Select the option menu “Choose an option”: you will see an option item “Request for an acceptance letter”. From there, you should be able to download the acceptance letter for visa application.