The United States sided with Britain because the German U-Boats attacked American merchant ships taking goods and persons across the Atlantic Ocean. British Royal Navy was very powerful and it blockaded Germany supplies of food and raw material. In response the Germans used submarines to attack supplies of Britain. The submarine warfare resulted in American civilian casualties.
In Jan 1917, Woodrow Wilson administration intercepted Zimmermann Telegram which urged Mexico to join the war effort on German sides and get back its lost territories form the United States.
The publication of the Zimmermann Telegram and the escalation of German submarine attacks on US merchant vessels led the US Congress to declare war on Germany on April 6, 1917.