Hitler’s early victories in WWII can be attributed to at least two factors.
First, Hitler had his country ready for war while the other countries were poorly prepared. Hitler used propaganda and other tactics to make the German people eager for war. He used their anger at the loss of WWI and their treatment under the Treaty of Versailles to make them want to get revenge and to regain their rightful (in their eyes) place as a major power. Meanwhile, the Allies did not want to have to fight another war. They were hoping for peace and were certainly not preparing for war. When the war came, Germany was ready while the Allies were not.
Second, Hitler’s military had come up with new strategies and tactics for fighting while the Allies had not. The Germans had lost WWI and had therefore spent a lot of time thinking about how to do a better job of fighting. The Allies were not as interested in thinking about a new war and they were inclined to be complacent anyway because they had won the last war. Hitler’s military came up with the set of tactics that we now call “blitzkrieg.” This was a new way of fighting based on rapid movement and close coordination between the arms of the services (air, artillery, armor, infantry) that had never been seen before. This gave the Germans a huge tactical advantage when the war started.
In short, Germany was more ready to go to war and it had better military tactics when the war started. These factors led to a string of early victories for Hitler and his military.