There was no one Pearl Harbor-like moment or event that pushed the United States into World War I. Instead, it was a combination of things that culminated in President Wilson asking for a declaration of war in early April of 1917.
One of the United States’ main issues in this war was the desire to trade with Europe. Trade with Britain, in particular, was very important to the US economy. For this reason, it had been opposed to Germany’s policy of unlimited submarine warfare. Germany had backed off this policy to avoid antagonizing the US, but at the end of January 1917, it decided it had to resume that policy. This upset the US.
The next event was the discovery of what is called the Zimmermann Telegram. This was a message from Germany to Mexico requesting Mexican help in the war. The Germans wanted Mexico to enter the war on their side to prevent the US from getting fully involved in Europe. They promised to give Mexico back much of the territory it had lost in the Mexican-American War if it participated in WWI and if the Central Powers won. This, not surprisingly, angered Americans greatly.
But even this did not actually cause the US to enter the war. During March of 1917, three American merchant ships were sunk by German submarines. These sinkings ended up as the last straw and Wilson asked for a declaration of war.