Minecraft wiki entry for Curing_a_zombie_villager, associated text: 1 Zombie Villager
Either 1 Splash Potion of Weakness, or 1 Lingering Potion of Weakness if you want to heal multiple zombie villagers, or 1 Arrow of Weakness, or 1 Witch to throw a Splash Potion of Weakness. You can also use a command to give the zombie villager the weakness effect.
1 regular Golden Apple