Mark 1:4-20 describe the ministry of John the Baptist and the beginning of the ministry of Jesus.
John the Baptist preached in the desert/wilderness near the southern end of the Jordan River, where it empties into the Dead Sea. He was preaching mainly to the people of Judea (southern Palestine) and its capital Jerusalem (1:4), but he was also on the main road that led from Galilee in northern Palestine to Jerusalem. It is interesting that John was in the wilderness, not in Jerusalem. The people who were interested in meeting God had to come out to him. No revival in history has ever started in a church headquarters.
It was also perhaps safer for John to avoid the cities, although this did not stop Herod from throwing John in prison. This was Herod Antipas, who inherited Galilee and Peraea (on the east side of the Jordan River) from his father, Herod the Great. This suggests that John was living on the east side of the river, even though he was drawing most of his followers from Judea on the west side of the river.
Jesus came down the road from Galilee and was baptized by John, but then went into the wilderness rather than continuing on to Jerusalem. Among other things, this kept Jesus out of the way when John was arrested.
Jesus centered his ministry in Galilee, in the north, although he made several trips to Jerusalem, especially for the Jewish feasts. Thus, while John prepared the way for Jesus, he did so by preaching to a somewhat different group of people. However, when Jesus went to Jerusalem to be crucified, the church of believers that resulted was first based in Jerusalem and Judea, among the people first impacted by John. The church spread rapidly in both Judea and Galilee, among people who had been prepared. Jesus, in the beginning of his earthly ministry, preached a message similar to John’s. John preached “a baptism of repentance for the forgiveness of sins” (Mark 1:4). Jesus preached: “The time has come. The kingdom of God has come near. Repent and believe the good news!” (Mark 1:15)