Virkon S. You can order online. Wash the thing with dish soap, then after it dries use a Hudson sprayer and soak it with the Virkon S. Don’t use bleach, the smell can be overwhelming.I'm running across problems turning the shed into a chicken coop, mainly in attaching a run. Yesterday the ground got extended so there'd be room before the dropoff where a run could be built, but the soil is not solid enough to where I'd think a run would last for very long. There's still a dropoff that could erode back toward the pen. Then there's an apple tree that would prevent putting a top on the run, which I'd need with predators. I've seen coyotes, fox, and a hawk, and there's a bobcat nearby.
With eggs in the incubator that will go into lockdown tomorrow, I wanted a coop. So I checked for ads, and found a used one of these for sale for $275. https://roostandroot.com/product/round-top-backyard-chicken-coop/#1-choose-standard-or-xl-model-coop
He's supposed to deliver it this afternoon or evening. If I ever get the shed turned into a coop, this one can be used as an extra or quarantine coop. Now I need to know the best way to sanitize it before use.