Like most countries who are at war or preparing for war, the Nazis put together a major propaganda campaign to convince people of the rightness of the war. In the case of the Nazis, there were two main targets of their propaganda.
The first target of the propganda was foreign enemies. The Nazis emphasized the idea that foreign countries were holding Germany down and keeping it subjugated under the Treaty of Versailles. They also liked to depict the alleged suffering of ethnic Germans in other countries. Both of these stoked animosity towards the countries that Germany ended up fighting in the war.
Second, the propaganda was aimed at convincing the German people of the danger posed by Jews. The Nazis wanted the people to support them in their purges of Jews and they wanted to play up the alleged threat to their country from Jews both inside and outside the country.
In these ways, Nazis used propaganda to encourage the German people to back their preparations for war and, eventually, to back the war itself.