There is lots of information out there for a great science fair project!
You can start with the AIRNow home page, www.epa.gov/airnow. Click on "Kids" and "Students" for a lot of kid-friendly information (including animations and games!) that will teach you about air pollution, how it affects your health, and what you can do about it.
Also try EPA's main page, with links to all kinds of topics for a great science fair project...http://www.epa.gov/