Hi Tore and welcome to the forum.
For a garter enclosure I recommend a 20 gallon aquarium or the equalant. A 10 gal. will do fine if it is a small speciman. I like to have a minimum length to equal the length of my snake. It will need to be in an area with a 75-80 F. temp or have some type of heat source, hide spots and it's very important the enclosure be escape proof. A water bowl can be shallow, but needs to be something not easily tipped over.
Hope this is some help.
Sid