The U.S. was already aiding the allies with supplies even before joining the war in a combat role. Hitler probably figured declaring war on America would allow him to use German U-boats to attack American supply ships and disrupt the allies supply chain.
1. We would've taken over the world, and no one would have heard of the Holocaust. Lots of countries do that stuff all the time, you only hear about it when they need something to convince you that country's evil. Not defending it, just saying. 2. They didn't, America declared war on Japan who Germany was allied with.
Assuming from your question that the U.S. was always an ally of Germany, Germany would have kept all the countries they invaded and probably would have been able to cross the Channel and attack England. Russia had to be begged to enter the war and provide a second front to weaken the German Army. They probably could have been kept out of the war, at least for awhile, with land concessions on their western border and in the Pacific. Germany would have very little inventive to attack them if they had access to the US's oil supplies. Without the US supplying the Allies, they would have collapses much earlier. And once the US entered the war, it's blockade and attacks on German shipping were instrumental in Germany's defeat.