When starting to train your pit bull with positive reinforcement, I find that the use of treats works the best. You may be able to try using a favorite toy. I try to wing my dogs off of treats and on to toys once they get older.
To train using positive reinforcement is quite simple. For the first couple of sessions with your dog, you are going to have to vocalize a command and then use the treat to trick the dog into doing it. For example I would say "Sit" and then hold a treat over the dogs head until the dog sat down. Then I would give the dog the treat while verbally praising her.