Welcome to the forum.
Here's a link to the care sheet which will answer many of your question.
Can you tell us which part of the country you are from?
This way we might be able to tell you which garter snake species you have.
Is this a store bought snake or a wild caught?
Real, safe fish and alive night crawlers/worms are the way to go if you don't want to feed frozen/thawed mice. The care sheet will also give you some ideas.
Looking forward to seeing a photo of your snake.
Why did you get a garter snake? Great idea but want lead you to this decision?
Garter Snake Care Sheet - Caresheets