Go to the following link:
HippoCampus - History & Government
OR
Follow this Path: hippocampus.org--->History & Goverment(Under Social Sciences Section) on the left hand side. Then Click on any one of following:
American Government
U.S. History
U.S. Government and Politics for AP*
U.S. History for AP*
Most relevent will be US History for AP and US History.