How do u train your dog to be nice to strangers?

i have a Aussie and healer mix and im the only one he isnt scared of he doesnt like kids or even my wife that much he is scarred of everyone. when people try to pet him he growls and or barks at them. and he nipped my sister in law. i dont want to get rid of him but i need to be able to trust him. i need some help.