Leeloo - just the fact that you asked that question probably means that your dog having pups would be a LOT more work than you anticipate. She can get pregnant during her first heat which can happen anywhere between 6 - 12 months (young females have a really hard time usually rearing pups.)
Breeding your dog, and caring for her and a litter of pups is not only EXPENSIVE but there are tons of complications that can happen that may endanger her or the puppies. Leave it to the professionals, trust me! Be prepared - you have to have a lot of cash to spend (for meds, food and spay/neuter of the pups) and don't mind keeping the remaining puppies that you can't find a home for for the rest of their lives, and the list goes on and on. That is, if you're going to be a responsible dog owner.
As I'm sure a lot of people here will tell you, the last thing this world needs is more pit bull puppies. Even if you can manage to place your puppies, and most can't place a full litter, the percentages are a few of them will end up in rescue and most likely be euthanized. You are better off spaying your female, not having any more messy heat cycles and helping the breed out as whole by taking good care of your little girl!