Take your kitten to its litter box as soon you bring it home. Sometimes that first visit is all is necessary. But until you're sure that the kitten understands, take it there after each feeding. Sometimes felines will not use a kitty litter box if something about the particular brand of litter is not to their liking. Sometimes they will prevent the container if its location is not private enough. Make an effort to solve these problems if the kitten does not use the kitty litter box right away.
The vet will also look at the kitten for symptoms of disease. Bring along sample of the kitten's falling so the doctor can look for worms.