Shoot zombies coming in through the window, and with a few rounds they will die when they hit the ground. If a zombie starts trying to enter, the containers blocking it will collapse, so you need to keep them away. You have two goals - thinning out the zombies, and stopping any from approaching the door into Jill's room. The best place to generally stand is around the counter, giving you a good view of the windows and the door. Head out, and after the cutscene, you need to defend the door against the zombies coming in through the windows. Before you head out, make sure you have a few clear slots in your Inventory, and have fully healed and reloaded your weapons.
