Hi Heather,
A 5" garter is definitely a newborn. The question is whether it was born late last year and managed to overwinter or if it was somehow already born this year. Has this been an unusually mild winter in Colorado, or has it been warm for a while now?
Starting off with a newborn at this time of year might be one of the most difficult ways to start. Snakes that were taken from the wild don't usually eat right away if there was a big temperature difference between outdoors and indoors. Sometimes it takes a week or two before they eat. Unfortunately, baby snakes have limited energy reserves, so don't let it go for more than about 2 weeks at room temperature without considering force feeding it (
http://www.thamnophis.com/caresheets...=Force_Feeding).
The first thing you need to do is to make sure the snake has the right environment in its enclosure. It needs plenty of hiding areas to provide a sense of security, temperature gradient from approximately 75-85 degrees (more or less, but one end of the enclosure should be warmer than the other), water, and a normal light:dark cycle. You can get some more tips here (
http://www.thamnophis.com/caresheets...itle=Main_Page).
Getting a newborn snake to eat can be challenging, but it usually helps to offer live food. You might try putting several small guppies in a shallow dish, or lid (not a lid from something smelly like pickles, though). Make the water just deep enough so that the fish have to disturb the water surface or flop around to create a lot of activity. They shouldn't be allowed to swim very efficiently because that little baby isn't a skilled hunter yet. The worms are a good idea, and you can also try cut up pieces of fish or baby mice (often called pinkies). You can offer them in a dish or with a pair of forceps to provide a bit of motion. Basically, you'll want to offer different foods at different times of the day until the snake decides that something smells good. Hope this helps,
Rick