With American Bulldogs, it's very important that you choose a good breeder that breeds for excellent temperament. They are known for being dog aggressive, especially same-sex aggressive. Some lines have questionable temperament with regards to people as well. They need a strong leader that will socialize early and often. They need lots of exercise, both physical and mental.
It's also of note that there are two lines of American Bulldog, and they can be pretty different in both appearance and temperament.
Johnson lines are more "bully", with the issues that come along with brachycephalic (short-nosed) breeds.
Scott lines are much more athletic, with the corresponding energy that comes with that.
Both lines are confident, loving dogs when trained, socialized, and exercised properly, but can be too much for some owners to handle. Take time to get to know the breeder and their dogs. Make sure you are ready for the responsibility that comes with a large, powerful, driven dog. Ensure that the breed isn't banned where you live, as it is often included in breed specific legislation. In other words, look before you leap.