Thank you for finding TCS to ask these questions! It's amazing, isn't it? You provide love, food, shelter - and they seem scared and/or ungrateful. When we first rescued the kittens, hubby kept commenting on how they just didn't seem to care.
In fact - this is not the case. They simply don't really get what's happening to them yet. Cats are very smart - but when it comes to living with people, it's not something that is pre-programmed in them to understand is a good thing.
The article to which Kelly referred does have lots of helpful info.
But the main thing is to think of this kitty as a wild animal that will come to trust and love you, but isn't there yet. She knows you feed her - but she's in a new territory (and cats are very territorial) and so she's scared. She doesn't understand yet that she's safe, and the most important thing from a people perspective is to realize the quickest way to gain her love is to earn her trust.
Like I pointed out earlier, cats that have been living outside don't come pre-programmed to love living with people. But once they understand they're safe, that their caretakers can be trusted, and that your home is their territory - and they no longer have to concentrate on survival - it's truly an amazing thing. They learn what play is. They know what it's like to be safe for probably the first time. They feel contentment - and express it by purring. And what an affirmation that is for people!
Depending upon the cat - meaning how old she is, what her other experiences/encounters with people were like, what she learned from her mother - earning that trust can be from a few days (kittens that were picked up and cared for by people at an early age - like anything under 8 - 10 weeks old) to a few years (an older feral - like 3 or 4 years old - that was physically abused by people).
More frequently, for a cat that's older than a kitten but hasn't been on its own too long, it'll be a few weeks until you start seeing her not hiding, and most likely within a few months she'll be "Queen of the Castle" so to speak. But one thing is for sure - it'll be on her schedule, not yours.
The best thing to do is just let her hide. Get her some toys, but don't necessarily try to play with her. If you find the toys you've left out have been moved around the next morning (at first she'll feel far more comfortable coming out and exploring her new territory when you're not around), then maybe try playing with her with a wand-type toy. Make sure you leave it in a place that is NOT accessible by the cat. Ferals are attracted to string and often eat it - possibly resulting in her needing surgery. Don't learn that one the hard way. When you move the wand, think like a mouse (or bird that lands on the ground).
There's many other tips - again, most of them are mentioned in the article in the link.
...and however contrary to nature it seems, the most important thing to do is ignore her at first. Just let her get used to the new environment, the new sounds, the new way of going to the bathroom, of getting water, etc.
When we first rescued the kittens, the more attention we gave them, the less interested they were in us. The more we ignored them, the more they wanted attention. With older cats it takes a little longer, but sooner or later she'll "get" that people are fun and make them feel good. But you've got to give that "survival" mode time to turn off. And since she was OK eating food you put out and coming inside, that "survival" switch is already on the move.
And to make sure you all remain happy, it's important to get her stuff that it is OK to scratch on. It is part of the physiology of cats that they have to stretch and scratch, so it's very important to provide a carpeted post, or the sloping cardboard scratchers - or the flat on the floor mats. We have all of them, and they each have different preferences. If you can provide all of them, that's really the best. But at least something, so she doesn't hurt your furniture.
It is also very important, as everyone has pointed out, to take her to a vet for a check-up. She probably does have worms, and she may have fleas or ticks. She really ought to have a rabies shot, and it is very important to have her spayed. There's a link to low-cost spay/neuter services in my signature.
You can also visit http://www.pets911.com/organizations/organizations.php
, type in your zipcode, and a list of organizations in your area will pop up. Not all of them will be "cat" organizations, but at least you can let your fingers do the walking (to find a free spay clinic or a low-cost clinic).
And of course, feel free to ask any questions!
Thank you for rescuing this kitty!