They Don't Grow That Large:

At birth, a garter snake will be between 5.5 and 8 inches long.
A full-size adult may grow to 3.5 to 4, feet, but a length of 3 feet or less is more common. Garters typically achieve half their adult size in the first year, and their full-size 12-18
months later. Their relatively small size makes them convenient for pet owners who do not have a lot of space.

They Are Relatively Short-lived:

Most garter snakes don't make it much past two years in
their natural environments because of predation. However, with proper care a captive garter snake can live ten years or longer, but many have shorter life spans.

Enclosure:
They Need A Pool In Their Housing:

In nature garter snakes spend a great deal of time by riverbeds and other areas near water. For this reason, their enclosures should have a pool where they can swim. A garter snake can be housed in a vivarium with a built-in swim area. An alternative is to put a bowl of water in the substrate of a garter snake's aquarium tank. In either case, the swim area must be large enough for the snake to be able to immerse itself completely in the water. The water level should be low
enough so nothing spills out on to the substrate when the snake enters the pool. Most species of garter snakes can be housed in a 20-gallon aquarium with a secure lid. Hide
boxes and a climbing branch should also be included in the enclosure. Strong and determined garter snakes are masters at escaping through even the smallest crevice, so their housing should be escape proof

Substrate:
They Do Well With A Variety Of Substrates:

Garter snakes will do well with paper substrates (butcher paper, unprinted newsprint, or paper towels) as well as with a mix of sterilized sand and potting soil, or dry mulch. Wood shavings are not highly recommend, since they can be ingested, cedar should never be used, since its oils are toxic to snakes.

Temperature:
They Need A Temperature Gradient:

Like all snakes, Garters are "cold blooded" animals that cannot regulate their own body temperature, so they must be
provided with an external heat source. Typically, this should be a heat pad placed under one-third of the enclosure. The garter snake's enclosure should have a heat gradient
running from about 70Fs at the cool end to about 84F in the basking area. Temperatures in the basking area should be allowed to drop at night. The temperature in a snake's
enclosure should never be "estimated," it should be read by a thermometer placed one inch over the substrate. A garter snake's enclosure should have two thermometers, one in the warmer area and one in the cooler area.

Diet:
They Eat A Varied Diet:

Unlike most pet snakes, which eat feeder rodents almost
exclusively, garter snakes take in a variety of foods. In fact, most subspecies will not accept feeder mice, and many will refuse to eat even pinkies. The dietary habits of garters
will vary by subspecies, but most will accept and do well on earthworms, other insects, and fish (live or frozen, such as guppies, sand eels and Lance fish). Wax worms and white
worms can be used to supplement the diet of many subspecies. Fish is not nutritionally complete, and should not be the sole diet of a snake.  Worms should be placed in a shallow bowl or jar lid to prevent them from picking
up loose substrate, which can be ingested by the garter snake. Important -- It is necessary to provide calcium and vitamin supplements as part of the diet. Please read the supplement section below:

The following is a possible supplementing schedule for this reptile. Supplements should also be used, since the garter snake’s normal choice of food isn't as nutritionally balanced as a feeder rodent. However, this is only a general suggestion -- the best specific schedule may vary depending on the supplement products used and other factors. Please consult product package directions, outside literature and a veterinarian to determine the optimal supplementing program.