In short
John was not the first to baptize, but unlike earlier Jewish baptisms, his was not to initiate non-Jews into Judaism (proselyte baptism), but symbolized the confession of sins for both Jew and Greek, to escape God’s wrath (Matthew 3:7).
No, the practice of baptism itself was not invented by John. Following the exile it became common practice among the Jews to baptize non-Jews who had converted to the Jewish religion (this was known as proselyte baptism). With John, however, baptism had a wholly different meaning. With him it was an external sign of the confession of sins, which was required not only of Gentiles but also of Jews in order to escape the just wrath of God (Matthew 3:7) and to share in the coming salvation.1
5 Then Jerusalem and all Judea and all the region about the Jordan were going out to him,