He believed that it was their fault Germany lost the First World War. In addition he needed something to cover his expansion (ie: instead of telling the public, "hey, we invaded France just 'cause I felt like it," he told them, "yo, Jews are unruly so we're invading France to keep a closer eye on them,".) He also thought that Jews caused the financial problems of the world because at the time it was a stereo-type that Jews were these 'penny-pinching bankers' that made bad decisions and eventually caused the world economy to collapse (not true, it was France being an idiot and demanding Germany pay for all her war debt and all of the rest of the allies' war debt (this is also one of Wilson's 14 points that France hated that would have fixed the world and most-likely averted the Second World War)).
Adolf was a very crazy man, you should search up all the other things that insane man wanted to do. He thought he was going to rule the world as a dictator, yeah as if the United Nations would allow that to happen, crazy nut, glad he's dead.
Anti-semitism existed long before Hitler. He used it as a way to galvanize the German people against their supposed oppressors who had embarrassed them in WWI. Sadly, many listened to him. The Western allies who demanded that Germany accept the blame for WWI didn't realize what resentment they were creating among the German people. The result was WWII. This is a very simplistic explanation but what do you expect on Ask.com?