Taylor should be your first survivor and you will encounter him shortly after leaving the diner and getting into the truck at the beginning of the game. You'll go under an overpass and an explosion on your right will blow open a path through a fence. Taylor's survivor icon will immediately appear just beyond the fence. Park the car, or use it to ram zombies, and completely clear the area, for Taylor to say thanks and run off. The achievement will pop once that happens. You'll get another chance about 100 yards down the road when you encounter Jodie, a woman stuck on the back of a semi truck trailer. Clear the area here as well if you somehow failed saving Taylor to get the achievement here (saving survivors is good PP, which is used for leveling, so I recommend saving everybody you come across).