why did alice get turend into a vampire and where was she when she got bitten [full story please]

Alice was held in a mental asylum because of her ability to have visions. There she was the favori of a vampire who worked in the building. James, a hunter vampire, took the challenge of hunting her simply because another vampire desired her. Once it was clear to him that James was after her, he, the vampire who worked in the asylum, freed Alice and changed her before James could kill her. James killed the other vampire instead. She had no memory of these events and was left to find her own way.

At some point she saw Jasper in a vision. He was searching for someone and Alice provided him with what he was looking for. They found each other in 1948 and par 1950 they had found Carlisle with plus help from her visions. They have been members of the family ever since.