how to stop my pitbull from fighting?

Step one is to spay and neuter all of them. Without excess hormones to escalate disagreements there will be less fighting.

Step two is to enroll the puppy (or puppies) in a puppy kindergarten class as soon as possible. There is a critical stage of development that occurs in puppies which will affect how they get along with other dogs for the rest of their lives. Without the right socialization during that period, dogs are likely to become dog or people aggressive.

Remote spray collars work real good in this case. I have used them before.