On December 7th, 1941, just before 8 am, hundreds of Japanese fighter planes attacked the US naval base at Pearl Harbor. More than 2,400 Americans died in that attack and 1,143 were injured. The day after USA declared Japan war and entered WWII officially.
Today Pearl Harbour is still an active US naval base, but there are four historical sites, all accessed from the visitor center. USS Bowfin Submarine Museum (at the visitor center), USS Arizona Memorial (reached by free shuttle ferry), Battleship Missouri Memorial (on Ford Island, take the free shuttle bus from visitor center), Pearl Harbor Aviation Museum (also on Ford Island, take the free shuttle bus). The visitor center has several displays and memorials, and is free of charge. So is the USS Arizona Memorial, but you will need to reserve tickets, as there is a daily limit. The last three historical sites all have admission fees.