Kimmi Gibbler

I adopted a two year old Pitbull Terrier. She came from a high kill animal shelter in Georgia. She must have been beaten because she'll flinch for no reason. Shes growled and barked at two little girls, that kinda scares me. She doesnt seem vicious in the house but once I take her for a walk shes a different dog. I want to prevent her aggressiveness towards other people. Any suggestions?