After completing the Sacred Silence quest, you'll be able to talk to Deanna in the church to start the quest.
You have to get three bottles of insulin from two stores and bring it back. There are a lot of free roaming zombies at this point so running past them is a good strategy. Get back to Deanna with the insulin to claim your reward.
Note: You may walk past the warehouse on the way, triggering the Simon Says... side quest