Hitler wanted to create a new Germany. He had many plans and projects. Some went as far as rebuilding the captial. All of these plans depended on a "pure Arian race" which meant the Jews had to eliminated. Remember that Hitler killed more than just the Jews during the Holocaust. He also targeted the elderly that he saw as weak. He targeted those who were simpathetic or tried to help the Jews. Basically, anyone who wouldn't fit into his new Germany was targeted.
Hitler believed that the Jews were impure and he hoped to cleanse Germany of their presence, since they impeded his master Aryan race ideals. Hitler also believed the Jews controlled too much of the nation's wealth, and eliminating them would give him more economic control.
I assuming you're asking why the Jews were targeted as victims of the Holocaust. The immediate answer to that is that they were targeted because Hitler saw them as a group that would hurt Germany and ruin its civilization. He believed that Germany would be stronger without the Jews.
As for why he believed that... it is hard to know, but it clearly has to do with the long history of anti-Semitism in Germany and other parts of Europe.