In selecting a common kingsnake for a pet, you should choose one bred legally in captivity, not one captured illegally in the wild.
Realize that the snake may grow to several feet in length, and will require some replication of its natural habitat. It may live for 20 years.
As Lianne McLeod, Doctor of Veterinary Medicine, said in "Kingsnakes and Milksnakes: Choosing a Snake," you should select a snake with:
A firm and uniform body with smooth movement, free of tremors
A glossy skin, free of sores and parasites (especially mites)
No open-mouth gasping for breath
Mouth interior uniformly pink, free of mouth rot
Clear eyes, free of discharge
Clean vent, free of swelling
Kingsnakes are "escape artists," said Melissa Kaplan in her Herp Care Collecting Internet site, "Kingsnakes and Milksnakes," so for an adult, you will likely need a 60-gallon enclosure with high sides and a tight fitting and locking top.