If you are traveling from outside the United States there are three types of visa for entering the country: tourist, work and student. We will focus on the tourist visa. If you want to obtain another type of visa, the best idea is to ask in the U.S. embassy in your country.
Citizens of the following countries that want to travel to United States for less than 90 days do not need a visa. The only requirement is a machine readable passport.
Andorra, Australia, Austria, Belgium, Brunei, Czech Republic, Denmark, Estonia, Finland, France, Germany, Hungary, Ireland, Iceland, Italy, Japan, Latvia, Liechtenstein, Lithuania, Luxembourg, Malta, Monaco, Netherlands, New Zealand, Norway, Portugal, San Marino, Singapore, Slovakia, Slovenia, South Korea, Spain, Sweden, Switzerland, United Kingdom.
To be able to arrive in the country without problems, it is necessary to fill out the following form at least 72 hours before arrival in the United States. The form should be filled out by all travelers, including children.
Citizens of other countries not included in the previous section should obtain a visa before the trip. For these visitors, the best idea is to consult with the U.S. embassy in your own country.
If you have any questions, you will find more information in the official website for traveling to the United States.