Genocide is a deliberate and systematic destruction, in whole or in part, of an ethnic, racial, religious, or national group.
To understand that It helps if you have read Mein Kampf by Adolf Hitler. While it is a very turgid read, Hitler was by no means a writer of any merit, the book does give a fascinating insight into HItler. From the end of the First World War several of the right wing(Nationalist) groups blamed the Jewish people for the loss of the war. A case can be made for genocide even at this early stage but it did not become government policy until after Kristalnacht (NIght of broken glass) in 1938. The laws Jeff mentions above were a big push in that direction.
Having said that, the Nazis wasted no time in setting up concentration camps after assuming total power in 1933. They really got into their stride after the Rohm purge. The death camps would evolve later after Germany occupied Poland. The laws got more restrictive as Hitler's regime consolidated power. Some believe that as the war went against Hitler then he started killing the Jews and other so-called "undesirables".I argue that Hitler had a detailed plan when he assumed the position of Chancellor and then Furher after von Hindenberg's death. He laid out a very precise plan in Mein Kampf and carried it out once he had the power to do so.