If you'll be coming from outside the United States, remember that Customs and Immigration processes at U.S. ports of entry have become increasingly strict; please make sure you understand what paperwork is necessary for travel from your country to the U.S.
Some countries participate in the Visa Waiver Program - from these countries, a visitor's visa is not required. See the U.S. State Department website for details:
If you are traveling from a country not participating in the Visa Waiver Program, then you will need a visa - and need to allow for sufficient time prior to your departure for visa processing - plan ahead! Some countries require an invitation letter in order to obtain a visa. If you need one, contact convention-info [at] origamiusa.org and we will provide you with an invitation to attend the convention.