Welcome to the forum. A great place to start is the care sheet:Garter Snake Care Sheet - Caresheets
Guppies and cut up night crawlers are good for babies. Earthworms also work well. I really encourage you to read the care sheet. It will answer many of your questions.
If you have a water dish for the babies that should do well. Just make sure they are finding it and using it. Offer food everyday until you get them established.
Once again, thanks for joining the forum.