If you have a passport from this list https://esta.cbp.dhs.gov/faq?lang=en (there is a visa waiver by applying for an ESTA only). You can apply for an ESTA here: https://esta.cbp.dhs.gov/disclaimer
Otherwise, you will need to apply for a B1/B2 visa here: https://travel.state.gov/content/travel/en/us-visas/visa-information-resources/forms/ds-160-online-nonimmigrant-visa-application.html