Caging can be a 10 gallon aquarium, make sure the lid is a tight fit because these guys are escape artist.
Substrate can be any wood based products just make sure it is small so that if it is ingested by your snake it can pass through safely. I have found that newspaper works great and easy to clean.
Your heat source can be an under the tank heater just make sure it is on one end because they need a cool place to hide. Daytime heat should be 85 degrees and nighttime heat should be 70.
Put in a water bowl which is heavy duty so that it doesn’t tip over, Garters tend to defecate in their water bowls so check daily and change, place that water bowl on the cool end of the tank.
To finalize your snake’s setup place a hide box or a piece of newspaper in their cage, put one where your heat source is and one where the cool side is. Make sure you put a couple rocks so this helps the shedding process.
Diet is varied they are opportunist by nature and are known to feed on fish, worms, frogs, toads, lizards, birds and rodents. Baby garters can be started on earthworms or superworms then as the snake grows so does the food. If your snake is fed on a fish or worm diet on a long term basis I would highly recommend complementing their diet with a vitamin supplement. If your snake is fed on all rodent diet vitamin supplements are not needed, feed adults every 7 to 8 days, feeding more frequently can lead to obesity and ultimately a shortened life span.
Hibernation is not mandatory for babies in their first year of life but after the first year to put them in hibernation turn the heat source down to 55 degrees with a 5 degree fluctuation being ok. Place them in complete darkness and provide them with fresh water and check on them periodically. Stop feeding them about 3 weeks before you start this process so that they can clean out their system of all fecal matter.