You will not need a visa to enter the Argentinean side of the falls, but Americans and Canadians will need a visa to enter Brazil. Most Americans just visit the Argentinean side. It is bigger than the Brazilian side and you get much closer to the falls. However, the Brazilian side is also great (the landscape view from the Brazilian side  is fantastic) and you may want to get the visa.

The visa will cost you around USD 110 per person and you can get it in 3 days in Buenos Aires before your trip. We can help you with that of course.

Please be aware of agencies saying that the visa is not necessary. It is, and you will be stopped in the border if you do not have one. Even when it is true that sometimes they do not check, it is also true that most of the times they do. We strongly recommend to get the visa if you are planning to visit the Brazilian side.