Obviously, get all the poop out; scrub the roost, nest boxes, etc. till they look as close to brand new as you can get them. Clean out all of the bedding (ALL of it) and spray either some bleach or vinegar on the floor, walls, and in any cracks that may exist. That will get out any bugs that may have been living in there. Since the newbies won't be living in it for awhile, leave all the windows open and if weather permits, let it freeze. Freezing it will make sure anything microscopic living in there will die. There will be others who are way more specific than me i.e., people who breed chickens and know heaps about biosecurity.
BTW, this isn't the time of year to be ordering chicks. You should wait till spring, which is when they are usually born. Then it is easier to keep them warm and they can mature at a more natural speed. Typically, they hatch around april or may, and start laying aroaund august. That gives them enough time to get into the swing of laying, and then they get a winter break. Then they will lay for longer, and their bodies won't get stressed out by making so many eggs so soon.
