Here is everything you need to know:
Vehicle Importation and Certification Requirements
This section is specifically just for Canada to US imports:
http://www.nhtsa.gov/cars/rules/impo...da03262010.pdf
If the vehicle is older than 25 years, it is pretty straight forward. Less than that and you will at least need a letter from the manufacturer citing its compliance with US standards. The links I provided tell you what needs to be in that letter and also had contact information for all the manufcaturers as well as checking what vehicles are pre-authorized for import.