Not real sure on the slate, if its just like any other rock going in an aquarium, i'd say hose it down and maybe brush it some to make sure its clean. Then let dry in the sun for a few hours?
As said, scrub and rinse thoroughly. You can use a mild bleach solution if you like, just rinse thoroughly and allow to air dry afterwards. Avoid soap or detergent; there will always be some residue left which can harm your fish.