After electing to bat, South Africa were in early trouble at 4-55 with the ball nipping around in the fresh morning conditions, but Pretorius, 19, put on 95 for the fifth wicket with fellow debutant Dewald Brevis (51) to change the course of the innings.
He said: "It was amazing. I was quite nervous (when I walked in). I'm glad (the debut is) over. We knew they only had two seamers and we knew we could take the spinners out of the attack. I just love batting, fortunately it worked out for me today."
Bosch's late burst provided a first test century for the all-rounder as he finished unbeaten on 100 from 124 balls and will resume on the second day with Kwena Maphaka, who has nine.
The touring side have only four starters from the side who won the World Test Championship final against Australia at Lord's this month, with several players rested and captain Temba Bavuma unavailable due to injury.
Pretorius's153 came from 160 balls as the left-hander became the youngest South African to score a century on a Test debut, hitting 11 fours and four sixes.
Zimbabwe twice felt they had him caught behind, but with no third umpire in operation for the two-game series, their vociferous appeals were turned down with no way to challenge the decisions.
Seamer Tanaka Chivanga got early joy for the hosts with an excellent spell of bowling, picking up three wickets in the first session and adding the prized scalp of Pretorius for figures of 4-83.
The South African youngster tried to pull a steep bouncer from Chivanga but succeeded only in skying the ball to Wellington Masakadza at mid-on.
Chivanga removed openers Tony de Zorzi (0) and Matthew Breetzke (13), and David Bedingham (0) in his first spell, bringing Pretorius to the wicket with the score on 23.
Wiaan Mulder (17) looked in good touch before he was run out chasing a quick single, but Brevis and Pretorius shared a quickfire 95-run partnership in 86 balls to swing the momentum South Africa's way.
Kyle Verreynne (10) came and went cheaply, before Pretorius and Bosch added 108 for the seventh wicket to put the tourists firmly in control.
