There are several considerations:
Health: A top priority is a visit to the vet to check for any health issues (illness, parasites) and get any immediately-needed vaccines (your vet will advise on this). The vet should also check for a microchip; if for any reason the vet can't check this, call a local shelter and ask where you can have the cat checked.
Do not allow the cat to interact with any other pets in your house before being cleared for health issues by a vet. When I adoped a stray from the street she lived in the basement for a few days, well-separated from everybody else -- not much fun for her, maybe, but better than living on the street or potentially infecting my other cats with something contagious.
Try to find the owner, if one exists and wants to be found: I mentioned checking for a microchip above; other things you can do, as JoshDM mentioned in his answer, are checking for and posting lost-pet notices. It's also useful to call all local shelters and the animal-control division of local government to report the found cat. An owner who's lost a pet will be calling those places, so this maximizes chances of you finding each other.
House prep: If these are their first cats, they'll need to do the "cats 101" stuff -- food, litterbox, removing (or putting away) household hazards, etc. That's beyond the scope of this question.
Integration: If there are other pets in the house, follow the usual practices for gradually introducing the newcomer to them. (I answered a related question about moving house here; I can't find a general "how to introduce cats to each other" question.) If the cat is feral (as opposed to one who was previously a pet) things can be more complicated; see here for some advice.