Garter snakes can be acquired during the spring or summer if you are goin to catch one. Look under boards or flat logs, or any other covered place a snake could hide. During spring, you can find communal dens where hundreds of snakes gather to hibernate. Or you could just buy one from the pet store.
The cage should be a 20 gallon glass aquarium or a wire cage with a secure, fitting lid(snakes have a unique ability to squeeze through impossiply small cracks and holes). Make sure you have a good substrate, bark chips will do fine(caution: Pine is deadly to garter snakes). You need a water dish full enough to fully emurse the snake in. You will also need a place to hide a hide box.
Food list: Earthworm(do not buy them), Small Mice, Fish(do not feed very often), and "Garter Grub"(sold buy T-Rex). Feed adult snakes twice a week. To feed earthworm and small fish, either dangle them in front of it's nose, or (for fish only) put them in it's water dish.