A 10 gallon tank will work fine. Make sure it has a secure lid! A small water dish, a hide box, and a rock to help with shedding. I use paper towels and newspaper for substrate for easy care and maintenance. If you use aspen or something like that, you should feed in another tank as fish or worms are wet and substrate will adhere to the food and could cause blockage in the snakes intestinal tract. You can check out my site....
Welcome to Don's Garter Snakes and Scott Felzer's site (reeeeeeal good one!)
Scott Felzer's Garter Snakes, specializing in aberrant garters Alan Frnacis has a real good site for info....
*Garter Snake Home Page - Alan Francis