Some resources to help you get started:    
                                                    
http://www.epa.gov/epaoswer/osw/kids/pdfs/sciencefair.pdf

http://www.sciencebuddies.org/science-fair-projects/project_guide_index.shtml

http://www.juliantrubin.com/environmentprojects.html

http://www.metrokc.gov/DNR/kidsweb/solid_waste_main.htm