Pit Bulls are not guard dogs. They are naturally friendly towards people. Like any animal they will usually protect their family when they feel it is necessary but you should not expect the dog to be aggressive.
I believe it is irresponsible of you to put the weight of protecting your family on your dog. It is your job to protect your family, dog included.
You must train your dog responsibly. As the owner of a Bully Breed you should hold yourself to a higher standard and put the necessary work in to ensure your put is well behaved and responsive to your commands.
This website has a lot of good content on responsible Pit Bull ownership including training techniques and background information on the breeds history that you might find useful. I suggest you use it.