Talk to Reevor by the Candlekeep Storehouse, and he'll pester you to kill the rats in the storehouse. Go inside and do just that. Come back out and talk to Reevor again to get your reward: 5 Image:Goldpieces.png and 50 xp. In addition to the quest rewards, the five rats that you kill are worth 1 exp each, and there is a barrel inside the storehouse that can be looted for a Silver Ring.