I think it is natural or you rat to be scared when you first bring it home. Also you bought her form a shop so there is no guarantee that she has been handled when she was a kitten. In future I would suggest finding a breeder, as responsible breeders choose calm and friendly rats to breed from and handle them when they are very young.
Is she on her own are there do you have any other rats? They don't really like to be alone so if she is this might be making her more anxious.
I think it is best to leave her alone for a while then gradually get her used to people being in the room ignoring her. Just get people to walk in, not look at the cage and just move about chatting so she can hear your voice. This way she will hopefully just get used to people being near in a non threatening way. Do this for a couple of days until she doesn't freeze when you walk in.
Then move onto every time you go near the cage giving her food. Therefore she will associate you with good things. I'd do this for a week or two until she isn't nervous about taking the food from your hand.
From there move onto stoking plus food and then light grasping around her midrift as if you are going to pick her up but don't. Another way is to try and get her to put her paws onto your hand and rewarding her, coach her gently onto your palm.
How to hold a rat? They are one of the easier small mammals to hold as they are that little bit more robust, it's pretty much the same as how you would pick up a hamster, except you have to give their bum a bit of support as well. Don't use their tail (ouch!).
Here is a nice description
Critters Collection: Rat
Another good 'trust training' description
the Rat Report
More good advice.
Getting Acquainted with Your New Rats
Why does she like you?
You have a mouse as well don't you? You probably are just better with small animals than your family! Some people can just get along better with animals. I think it is down to patience and calmness.
Hope this helps :)