What Karen said is right. Time and patience are the key. This little girl sounds like she might be semi-feral, or even feral.

How old is she? Put her in a quiet room with a litterbox, food and water along with a night light and possibly a radio with classical music playing. Go in with treats (she'll most likely hide) and sit on the floor, gently tossing them to her from where you are. You need to get down to her level. Cats feel threatened (especially ferals or semi-feral) when they come upon a human standing up. When you are down on their level, it's less threatening. Slowing blink at the kitty, which will let her know that you aren't a threat. Keep tossing the treats, avoiding any sudden movements. If she comes out while you're there, just allow her to come out on her own terms, gently talking to her.

I'm so glad you're taking this precious baby in. I'm sure there are other guru's here that can add some wonderful, helpful advice.

Good luck!!