Candy is this games currency. It can be used to buy costume upgrades and Creepy Treat Cards from a shopkeeper named Shady.
Ways to get candy:
-Bashing breakable objects
This achievement should come naturally as you play through the game, as long as you collect as much candy as you can from breakable objects throughout the world. If you somehow miss this achievement, there are some Creepy Treat cards that give a boost to the amount of candy you win in battles. Use those at the infinitely respawning Monster Patrols at the Dental compound until you've earned enough candy.
Also keep in mind that there is a limit to how much candy you can hold at once, so be sure to spend them when you can. You should prioritize the Superhero, Wolfman, and Hot Dog costume upgrades, and Creepy treat cards, because those items are related to other achievements.
There are 3 hide n' seek side quests for finding kids hiding throughout the game. Completing these quests will upgrade your candy pail, increasing the maximum amount of candy you can hold at one time. See the videos below if you need help finding any of the kid locations.