As far as I remember, some of those 19 people took flying lessons in Florida. They upgraded their capabilities in order to fly those commercial airplanes. What exactly happened in the cockpit, we don't know, but they hijacked the airplane.
I don't know if they forced the pilot who was sitting in the cockpit to fly the airplanes into the World Trade Center and the Pentagon or if they replaced them. As far as I remember from the media and the press, some of those 19 people, led by Mohamed Atta, took flying lessons at flying schools in Florida.
By the way, if you want to take flying lessons in Florida today, you have to undergo something like NEXUS registration to verify your personality and so on.