Rat Terrier afraid of leash?

I just adopted a female rat terrier from a shelter & I have only had her about 2 weeks. The vet said she is between 8 months to a year old, so she is very young still. I think she has had some training, because she is already house broken & has only had a couple of pee accidents. I go out with her and she stays close even though I don't have a fence. I have been trying to let her stay out a little longer each time so she can run & play. But today she would not come when called. She is just getting use to her name, but she would always come when I was heading back into the house until today. So the next time out I tried to take her on a leash, but I had to carry her. Then I hooked her to a yard stake. She didn't like any of it and was freaking out. I think somebody used a choke collar on her. It was pitiful because it almost undid all the trust & love we have built up over the past two weeks. So my question is How do I train her to a leash?