Some nationalities benefit from the Visa Waiver Program, refer to the updated list. You must then apply for an ESTA.
WARNING: ESTA must be requested several days BEFORE you arrive in the USA and it is mandatory.
If you are not eligible for the Visa Waiver Program, you must apply for a B1 (tourism) and/or B2 (business) visa.
WARNING: If you visited certain countries before you came to the USA (see FAQ of the Visa Waiver Program), the ESTA is not valid and you must apply for a regular B1 (tourism) and/or B2 (business) visa.