Not everyone needs a visa to visit the United States. The United States has something called the Visa Waiver Program (VWP) that allows citizens of 38 countries to enter the United States visa-free. Many western countries and developed economies worldwide are on the Visa Waiver Program list. If you are a citizen of a VWP country, then you do not need a visa; however, you still need to apply for an Electronic System for Travel Authorization (ESTA) before you visit the United States.
If you are not a citizen of one of the 38 Visa Waiver Program countries, then you will likely need a visa to enter the United States.