Depending on your country of origin, a passport valid for at least six months after your expected departure may be enough to grant you entry into the United States for up to 90 days. Visa waiver countries include most European Union members as well as Singapore, Japan, South Korea, Australia and New Zealand. Check your country's status at the U.S. Department of Homeland Security website.
You may require this visa if you have a temporary interest, such as a conference or medical appointment, in the U.S. To obtain the visa, your passport must be valid for at least six months after your departure, and you must show evidence that your stay will be temporary. Additionally, you must be "admissible" to the U.S. in compliance with the Immigration and Nationality Act.
The other short-term visa you may require is a confirmation that your reasons for entering the U.S. are tourism or family- and friends-related. As with all visas, this requires your passport to be valid at least six months after leaving the US. You should be prepared to prove to immigration officials that your stay is temporary, that you can support yourself during your visit and that you have no intention of abandoning your home residence.
If you intend to stay for more than 90 days, or for a purpose other than temporary business, medical or travel reasons, you will be required to apply for the appropriate student, working, internship, marriage or other visa. Each of these has its own set of rules, requirements, paperwork and fees.
