A garter snake is like any other snake in behavior and habits. Snakes are relatively docile by nature and easy to care for. Since you know what they eat, one obstacle is out of the way. I've kept a few reptiles before and so have some friends of mine. Tips for their comfort (and safety):
1: Use a heat lamp, not a hot rock. Monitor room temperature. 78 - 85 degrees fahrenheit is a good temperature range. Put an aquarium thermometer on the outside of their terrarium and keep an eye on it.
2: Keep fresh water available. For a snake, a shallow pan or dish full of water is a good choice.
3. Once a day, apply moisture to the reptile's skin. A fine misting bottle (the kind used for plants) works great.
4. Garter snakes, when stressed, can release a foul smelling, greasy substance. So, try not to stress the snake if at all possible.
5. Remove fecal material from the terrarium as soon as you see it (or smell it

).
6. Examine the reptile closely from time to time, paying particular attention to region of the head and colaca (where the anus is located). Look for mites, crusting, contusions, or anything else unusual. With a reptile, prevention or catching a problem early is about the only way to keep them healthy. They don't have a good track record of surviving illnesses and infections.
Snakes make pretty cool pets. When you raise them from hatchlings, they don't mind being handled by others and are surprisingly calm. They usually like to coil around your hand or arm. The biggest downside to keeping a snake (and any other carnivorous reptile, for that matter): their doo-doo smells awful. Yecccch.
Keep an iguana. They're vegetarians and make little cow-patties.

(I've also heard that iguanas don't taste like chicken)