Considering the message of John the Baptist and the Jewish leaders reaction to it, what was the existing Jewish thought about Baptism? Who needed to be baptized and when?
The Jews in and even before John's time didn't actually call it baptism, they called it "Mikvah" (or "Mikveh"). It was used for purification for women after childbirth and menstration, for men after they ejaculated , to prepare a body for burial AND it was also used as a ritual for people who were converting to Judaism !
In the case of John the Baptist, he was converting people, getting them ready for Jesus!
what was the existing Jewish thought about Baptism?
Who needed to be baptized and when?
According to the German Wikipedia, baptism was not common in the time of John the Baptist among the normal Jewish people at all.
There were different smaller (and probably more radical) religious groups (like the Essenes) that practiced rites similar to baptism.
... the message of John the Baptist and the Jewish leaders reaction to it ...
The reaction of the Jewish leaders is surely caused by John's message, not by the rite of baptism itself:
According to Mt 3,7, John the Baptist explicitly accused the religious Jewish leaders of his time to live in sin.
This explains that the Jewish religious leaders definitely did not agree to his message at all.