Most people in the United States initially did not want to get involved in World War I but several factors pushed the country into the war. First, most Americans were sympathetic to the British cause. We had much in common with them such as language and literature. Most Americans also did not trust Kaiser Wilhelm, who was an autocratic ruler. The issues that really pushed America into the war included German submarine warfare. After the Germans sunk the Lusitania, a British passenger ship including 128 Americans, the United States expressed its outrage. Although the Germans promised in the Sussex Pledge in 1916 to at least warn ships before shooting at them, they did not keep their end of the bargain and no ships were safe in the Atlantic. A second reason was the Russian Revolution which allowed Americans to be more comfortable about a war alliance that included Russia. Most Americans had abhorred the autocratic Russian State and did not want to work in an alliance with them. A final reason America entered the war was the publication of the Zimmerman letter which proved that Germany was taking a strategic interest in attacking with the United States. It did not matter that Mexico was too torn by internal strife to be interested in Germany's offer, it only mattered that the Germans were interested. In April of 1917, the United States officially declared war on Germany.
An excellent answer, the sinking of shipping and the Zimmerman telegram were the overt reasons America got into the war. The Lusitania incident was the most widely publicized, a little ironic when you consider that the Amercian government and the major newspapers all knew that the ship had been carrying 200 tons of munitions to Britain, thus making the Lusitania a legitimate military target under the laws of naval warfare. But further shipping losses and the lives of civilian passengers, including both American and other neutral citizens, led to widespread outrage.
The underlying reason for eventual American involvement , however, was the same thing that led Britain into the war. The British recognized it even before the war began, and President Wilson reluctantly shared the same view; a German victory over France would leave Europe dominated by a militarist power, and that simply could not be allowed. The long-term consequences would have been too dangerous.