It is not completely clear why the US entered WWI. There was no single event (as would happen with the Pearl Harbor attack in 1941) that obviously pushed the US into the war. Instead, President Wilson asked for a declaration of war for weeks after any event that is typically cited as a cause of US entry into the war. In general, though, historians typically say that there were two main reasons for American entry.
First, and perhaps less important, was the Zimmermann Telegram. This was a telegram sent by Germany to Mexico and intercepted by the UK. The telegram was given to the US on February 24, 1917. In the telegram, Germany asked Mexico to join the war on Germany’s side. In return, Germany proposed to help Mexico regain the lands that had been taken from it in the Mexican-American War of the 1840s. This was seen as a major insult and a hostile action by Germany.
Second, there was the issue of unrestricted submarine warfare. Germany was trying to strangle the UK by sinking ships bringing supplies to that island nation. Germany felt that this gave it its best chance to win the war. Early in the war, Germany had sunk both merchant and passenger ships without warning. To avoid conflict with the US, however, Germany pledged to avoid sinking passenger ships and to warn merchant ships so their crews could evacuate before being sunk. By 1917, Germany had decided that these restrictions were making it impossible to sufficiently curtail shipping to the UK. Therefore, Germany announced on January 31, 1917 that it was going to resume unrestricted submarine warfare.
Although these are the most commonly cited causes of America’s entry into WWI, we must note that Wilson did not ask Congress to declare war until April 2, 1917. This makes it less easy to say that either Germany’s resumption of unrestricted submarine warfare or the revelation of the Zimmermann Telegram was the cause of American involvement.
Once the US became involved, its entry was fairly decisive. The Allies and the Central Powers had fought to a stalemate. When the US entered the war, it brought in huge numbers of soldiers who could eventually be thrown into the battles and tremendous amounts of material support for the Allies. This tilted the balance towards the Allies and the Central Powers were forced to surrender a few months after American troops actually started fighting in Europe.