How do i make a scared dog unscared?

Ok yesterday we got a dog from the pound (It's a girl)and shes terrified we do think she was abused and ive been doing nothing but pet her and try and walk her but when i do walk her she just runs and hides up under something so then i have to drag her out by her leach.And when we let her in the house she runs under the couch or the bed.And my mom said that if she doesn't perk up by friday (its Tuesday)she going back to the pound and getting put to sleep so please help.