Wikipedia says that “despite the flood of new weapons systems unveiled by the British, Germans, French, Austro-Hungarians, Italians, and others in the war in Europe, the Army was paying scant attention. For example, it was making no studies of trench warfare, poison gas, heavy artillery, or tanks and was utterly unfamiliar with the rapid evolution of Aerial warfare.”
That seems batshit crazy. The Army literally sat on its hands and refused to learn anything from 1914-1918? Is this true? The Wiki doesn’t offer any cites; does anyone have some good ones?
The answer is simply, "Not prepared at all." America barely had an army of 100,000 men in April 1917 when it declared war on Germany. It wasn't an experienced army, either, not having fought a major ground war since the Civil War 50+ years previously. Despite the rapid mobilization for war, the US largely failed to provide its expeditionary force with tanks, planes, and artillery, so they had to use foreign equipment (predominantly French); even the helmets American soldiers wore were provided by the British. This was also partly necessitated by the need to save shipping space and send as many American soldiers to the front as possible, with the onset of the five German offensives from March-July 1918. American military tactics were also pretty antiquated and sub-par, with the reliance on mass frontal assaults (long since discarded by the British and French) and the poor coordination of artillery and infantry.
So the need to build a huge army from scratch, then send it on a trans-atlantic voyage, and then train and equip that army once it arrived, meant that the AEF wasn't ready to take the field for over a year. There were 300,000 American soldiers in France in March 1918, but 1.3 million by August and 2 million by the end of the war. Their first major operation was in July 1918 when 140,000 American soldiers counter-attacked the Germans along the Marne and repulsed what ended up being the last German offensive of the war. They conducted their first independent operation in September 1918 when they pinched the Saint Mihiel salient south of Verdun, then swung north and attacked in the Argonne forest with a French army on their left flank. US tactical inferiority (and unfavorable terrain in all fairness) showed here, with the AEF taking 120,000+ casualties in a month and a half of fighting against demoralized German soldiers.
tl;dr: America wasn't prepared to fight, shown in the time it took them to assemble an army ready to fight on the Western front, and when it finally did fight, it took heavy causalities in subsidiary roles. Still, with all this being said, Germany doesn't seek the armistice without the American military intervention; it was the decisive factor in the war, as Ludendorff, de facto German ruler, himself admitted.
Sources:
The First World War by John Keegan
The First World War by Hew Strachan