Check your local laws concerning collection of wildlife. Most states require a permit to keep wild animals as pets. What do you plan to feed your wild-caught snake? Do you think it will even take food? You may find that it is too scared to eat. Garter snakes do seem to adapt okay to captivity. They like earthworms and crickets. You've got to understand the needs of an animal before keeping it as a pet.
Have you looked into a baby snake from a pet store that was raised in captivity? These make better pets because they are used to human contact and will be perfectly content to be handled and fed by a human.