Welcome to the forum! These folks know EVERYTHING about garters!
There's a care sheet here that'll tell you about diet, etc.
They don't usually like crickets. Fish, worms, frogs are the main things. You can get feeder minnows (not goldfish, those are toxic) or use fresh fish from the supermarket, like trout or salmon.
I'm new to garters, myself, so I'll let the others give you the full run down
Glad to have you here!