how 2 train a dog that has been abused & is very skiddish

Hi my name is Amber Hoover & my mom bf got a 2yr old pit bull from a shelter & what they told him was her owner was an elderly lady & 1 day she put her outside on her chain & went back in 2 take a nap & passed away in her sleep & no1 found the lady 4 2wks & the nieghbors supposedly was taking care of the dog & found out they were actually abusing her she doesnt like men especially 1s that smoke bc u can tell she was burned w/cigs she has scars on her paws so since my moms bf couldnt get her 2 come 2 him he gave her 2 my mom & I live w/my mom the dog attached herself 2 me my brother lives w/us 2 & she acts like shes terrified of him & she barely goes 2 my mom so my question is Idk if u would kno but I would like 2 kno how would go about training her bc Ive never trained a dog especailly avery skiddish dog w/out terrifing her more & away from me I think she was used 4 breeding 4 a puppy mill bc shes had pups but now shes fixed so she might have been abused her whole life & Iam so nervous about scaring her even more so could u plz tell me how 2 handle her ty