They're really just looking to make sure your home is safe.
As far as having anything "ready" - we were in the process of renovating the room that will be the baby's room (we live in a fixer-upper & decided to do that room while we were waiting to be approved so it would be ready if we were chosen quickly or for a short-notice placement) - literally, no floor covering, spackled walls doors off the hinges & on sawhorses being sanded. Our caseworker told us it was a beautiful room - because of the light that comes in from the windows.
It's nerve-racking for sure, but it really is a piece of cake. We showed her that we had fire extinguishers on every floor, no uncovered outlets (we actually fixed that the day of her visit - like I say, it's a fixer-upper, and we hadn't finished those details in a few rooms, lol), that sort of thing. She actually had to phone us later on because she forgot to check for smoke detectors & she wanted to make sure we had them.
They might also give you some tips on things you might not think of that you may need to childproof. She asked us about our double staircase & how we planned to work the childproofing there. There may be things you never thought about that they'll point out so you can address them.