Are we talking from newly born? if so I have found a mix of chopped pinkies and a little chopped trout will usually be enough for any baby garter to start with, as they grow (very rapidly) go to half pinkies then full, eventiually dropping the fish all together, or giving as alternative food.

If I have difficult eaters, I give them the same chopped mix, but add a few small worms (or cut worm pieces), the writhing of the worm usually sparks interest, once it starts to eat the worm, it generally just carries on eating!

If you are trying to change over food, I have found adding a pinkie to the fish, well rubbed so that all the food smells fishy, gradually more pinkies, less fish, until you need no fish at all.