There are two possible answers to this question.
First, we can look at the actual immediate causes that led the US into the war. One such cause was the Zimmermann Telegram, in which the Germans tried to get Mexico to enter the war against the US. Mexico was to be rewarded with the land that the US had taken from it in the Mexican-American War. This naturally turned American opinion against Germany. More important, though, was the German resumption of unrestricted submarine warfare. The Germans badly needed to cut off supplies to England. They tried to do so by using submarines, which would sink ships without warning. Germany had promised to stop doing this, but then went back on that promise and sank American ships. This was the immediate cause of America’s entry into the war.
However, we might also look at deeper, root causes of the US’s entry. Basically, it was in the economic interests of the US to enter the war. The country did much more trade with England than with Germany. The British also owed the US a great deal of money by 1917. Many historians believe that these economic calculations also entered into the decision to get involved in the war.