Garlic is used for the Fishing Contest and Vampire Slayer quests. Despite being a food, it cannot be used in the Cooking skill, also can it be eaten to heal 15 hitpoints which is better than lobster.
Where to find garlic:
On a table in a house in Seer's Village.
On a table in a house near the marketplace in Ardougne.
In a cupboard in Morgan's house, second floor, in Draynor Village.
Buy or thieve from Greengrocer vegetable stall in Etceteria and Miscellania.