America officially entered WWII because it was attacked by Japan. The United States declared war on Germany on December 11 because Germany had declared war on the United States that same day. The declaration passed unanimously in both the Senate and House. Prior to entering the war, America had supported Allied efforts in Europe, particularly through the Lend-Lease program which help the British speed up munitions and armament production. It's heavily assumed that Roosevelt wanted to enter the war earlier than 1941, but it was fairly unpopular with the American public and Roosevelt wasn't certain he could convince Congress to declare war (hence finding ways to work around restrictions on the US government to support the war in Europe, like Lend-Lease).

While Americans were aware that Jews were being persecuted in Axis controlled areas, the general public wasn't made aware of concentration camps, death camps, or the Final Solution until 1942.

Edit: I should add that the Final Solution also was decided until 1942 as well.

Second edit: changed the German date