There are many reasons that can be cited for why the US entered this war. Let us look at three of them.
The factor that most directly led to American involvement in the war was the resumption of German unrestricted submarine warfare. The Germans had been using submarines to blockade England, sinking ships without warning. This was in violation of international law. It also angered the US because it led to the sinking of American ships. The Germans pledged to stop doing this, but later rescinded the pledge. This was the direct cause of the American declaration of war.
A second factor was the Zimmermann Telegram. This was a telegram from Germany to Mexico. In it, the Germans offered to give Mexico back the lands that it had lost to the US if Mexico would participate in the war. Publication of the telegram (intercepted by the British) led to great anger among Americans.
A third factor, and one which some say was the major underlying cause of US entry, was the fact that the US had strong economic ties to England that were getting stronger as the war went on. It had already traded more with England than with Germany, but the English blockade of Germany made the discrepancy much greater. England also owed a great deal of money to the US. Therefore, the US had an incentive, it is said, to ensure that England won the war.
These are three of the more important factors that caused the US to enter WWI.