Let me give a little context first. The United States did not want to enter WW I, but when German U-boats repeatedly sank non-warships, the United States formally declared war on Germany. As a side note, the Germans sank seven U.S. merchant ships. War was declared on April 6 1917.
At this time, the United States did not have a large army, but after the passage of the Selective Service Act, 2.8 million men were drafted. This was a huge influx of soldiers. Around 10,000 men were being sent on a daily basis to France. In addition, the United States worked it out with Puerto Rico to send Puerto Rican solider to the war as well. This meant more able men to replenish the war effort.
Another way the United States helped the war campaign was it sent ships. They sent battle ships and submarines to aid in the defeat of German U-boats and to reinforce the the naval defense.
Finally, all of this gave a huge spike in morale to continue in the war effort. In addition, this made the Germans more demoralized.