Most Canadian citizens and many citizens from Visa Waiver Program countries can come to the US without a visa if they meet certain requirements. Keep in mind that it might take some time for your embassy to process your application so make sure you do not wait until the last minute.
If you are visiting from a non visa waiver participating country, please go to the US Embassy website to determine the visa requirements for your country.
Visa waiver program
Travelers coming to the US for tourism or business for 90 days or less from qualified countries may be eligible to visit the US without a visa if they meet the visa waiver program requirements. Visit the Visa Waiver Program site to see if you are eligible.
Currently, 35 countries participate in the Visa Waiver Program, as shown here: