There were a few things he promised. He promised to unite the people and give the people of Germany back their pride and glory that was lost during the first world war. He promised to destroy/overturn the Treaty of Versailles (It put Germany at odds and in debt to other countries) and give back Germany the land that was lost during the war and bring her back to what she was before the war.
How he did this was violate the Treaty by raising an army and militarizing Germany. He then invaded neighboring countries stealing from them. Then he went on to destroy "inferior" races such as the Jews and Slavs.