There is a lot of books and topics here explaining it and the answer is pretty well known. You do not seem to have made a lot of research.

Basically the German defeat in WW1 has been pinned on a jewish conspiracy by several far right movements looking for a scapegoat. Hitler having fought in this war, where he developped a great nationalism and a real love for the military matters, got devastated and disgusted and adhered to this theory of the Jewish conspiracy.

After the first war, he worked for the military intelligence and got the mission to infiltrate a far right group of agitators and report back to his superiors on what they were up to. Once member of the group, he quickly fell in love with the ideas of the leader of group, Dietrich Eckart, partly because that was exactly what Hitler had always wanted to hear : the German man is superior and the WW1 defeat was because of the Jews and communists plotting against Germany in the rear while the real Germans were fighting on the frontline.

Hitler's oratory talents made the rest and made him the leader of the group really fast.

In Hitler's eyes the jews were parasites without a culture of their own, they would therefore cling to another culture, infiltrate it and then corrupt it from the inside. His dream of a pure Germanic Reich was therefore not attainable if there was jews to make themselves pass as pure aryans and corrupt it from the inside. The first solution was to oppress them to force them to leave to Palestine. When it did not work because a lot of jews considered themselves as German citizens and did not want to leave their country, the extermination began.