The United States prepared for WWII in a number of ways. Let us look at two of the most important.
On the one hand, the US prepared by helping Britain and the other Allies in ways that fell short of full involvement. The most important of these was the Lend-Lease program in which the US sent huge amounts of war materiel to England and the Soviet Union to help them in their fight against Germany.
On the other hand, the US prepared at home. In September of 1940, the US started drafting men into the military. This was the first time this had ever been done in peacetime. The US also started to greatly improve the quality of equipment being used by the armed forces.
In these ways, the US prepared for its own eventual entry into WWII.