This is one of those questions which requires a really long answer, but I'll do my best to keep it short. This was actually what my final thesis paper was about to obtain my degree so you are in luck.
At the end of WWI the German people felt betrayed by their own government because many believed the war was lost because of an infiltration of Jews who undermined the Kaiser's ability to rule Germany and wage war.
The Treaty of Versailles forced a heavy burden upon a war torn Germany which compounded the suffering of the German people. To make matters worse the Kaiser fled Germany to Belgium and denied any blame for the war. What solidified the idea that there was a Jewish-Bolshevik conspiracy against Germany was that after the Kaiser fled, he was replaced by Kurt Eisner who happened to be a Jew and was also very vocal when blaming WWI on Germany. The removal of the Kaiser's power and replacing of him with Eisner was called the Munich Revolution and was one of the influencing factors which caused Hitler to join the socialist parties.
Skip forward a few years to 1929 and the crash of the stock market; this plunged the entire globe into an economic depression from which there seemed to be no escape. Unemployment skyrocketed from 1 million to 6 million people without work and relying on the charity of others or the government just to survive. Hitler, being the master propagandist, seized upon this opportunity to increase his antisemitic rhetoric and convince the German people that the Jews were to blame for the high unemployment and general misery. He mostly directed this type of speech to the blue collar workers who were most affected by the depression and were also mostly undecided who they were going to vote for in the upcoming elections.
Cut ahead a few more years to 1932 Presidential elections. Hitler not only didn't win, he lost miserably and President Hindenburg was re-elected, but Hitler had a trick up his sleeve, he had been making powerful friends in the German industrial sector. Because these men basically controlled the German economy, Hitler was able to use his influence with them to force Hindenburg to appoint Hitler as the new Chancellor of Germany. Once Hitler had his foot in the door someone, though nobody knows exactly who, set fire to the Reichstag building in Berlin. Hitler used this act to, once again, force Hindenburg to hand over all power to him as a way to protect the German people. Once Hindenburg had done that, Hitler had finally taken total control of the German government and was able to begin implementing various polices which ultimately led to the Holocaust and WWII.
