There are two ways you can go out of your safe room in this level, but you most likely want to go directly ahead, into another room with boxes and shelves. Be aware of the rooftops as you go, you may get jumped. Check the room for molotovs, then move ahead. To your right is a lit garage - there may be weapons, goodies or healthkits here, so look around. Once you've got everything go straight down the alley, then left - keep your eyes open for items on the ground and infected on the roofs. On your right will be a door to a kitchen, enter it. The entrance to the restaurant is glass, so take this opportunity to shoot any unsuspecting infected you see. You will be in a truck dock of sorts - at the end of it is a gas station. You should shoot this now - it will draw attention to you, but if you don't, it could go up by accident and can be a real problem. What's more, it opens up an extra room which you can access via the debris - check there for weapons and items. When you're ready, activate the lift. You don't have to be on it, it has a ladder, but it's a good idea, since you want to move quickly here. Watch for smokers who will try to pull you off as you make your way along the roof. Once you reach the room at the end of the path, make sure you're all together before you drop into the hole. This room will have healthpacks and ammo, at the very least. Don't stay here for long though - infected will come after you, and smokers can pull you through the hole. You can also open the door to the dock from here if you left someone behind. As you move down the hallway, check for items in the rooms. Run quickly through the room with the boxes, down the hallway and into the stairwell.
You should be near a room full of machinery. The entrance to the sewers is near. There is one manhole in the middle of the room, and one in a room off to the side. The one to the side is safer, but the one in the middle is closer to your destination. Assuming you took the safe route, go down the sewer to the first left and follow it to the second tunnel entrance. This should be right below the other entrance. Follow this route - it will take you directly to a room with weapons and items. Be aware, this is another likely tank location. Follow another tunnel from this room. Upon exiting, the ladder to the surface will be to your right. This can be a difficult spot - special infected will likely be waiting for you at the top. Go all at once, and be alert. If you survive, the entrance to the hospital is right there.