I don't mean to be discouraging, but this question needs to be thought out more thoroughly before you start actually work on it.
If you mean pollution very broadly, it would be extremely difficult to measure this accurately yourself unless you have access to fairly sophisticated lab equipment, so this would be pretty much entirely a research project (that is: looking up the information in published resources).
I'm not sure how well that would fit with the expectations of your science teacher or the science fair.
It might be possible for you to measure one particular kind of pollution (say, ozone, perhaps), but answering this question with any accuracy would require sampling a lot of cars, because any one car (or small number of cars) might just be out of tune.
It might also be a challenge to define what you mean by "american" and "foreign". Most modern cars are built of parts from a variety of countries. You could of course, just define it by the brand, but for example, any Toyota Camry that you'd be likely to find in the US is made almost entirely in the US (in Ohio, if I remember correctly).
If a research project is what you wanted to do, then the EPA publishes the emission figures for every passenger car sold in the US. You can find this data at:
http://www.epa.gov/otaq/crttst.htm.