You have to make your way to the airport just outside of "Area 69" and go in to the building. (Note there are 2 entrances, you want the one that goes up the stairs) In there you will find a mission on flying, which takes you through various courses on the Airplane and Heliocopter, as well as Skydiving. The flight dynamics on GTA:SA suffer the same poor play-ability as the other games which CAN make getting your licence a HUGE pain. I found the barrel rolls through the circles to be very tough! Once you have completed all of these, you will have your license and it is time to wreak some havoc!

You can also get a pilot's license by stealing a plane and go flying. If you just started the game and can't get to Area 69, you can sneak into Los Santos International Airport. To do so, you need to steal one of those auto transport trucks with the sloping trailer. Park it adjacent to the brick wall where the pilots entrance is on the north end of the airport. This will let you walk up the slope of the truck and jump the wall. The inside of the airport is HUGE, but there's two airplanes parked there (you'd think a major international airport would have many planes all over the place, eh?). There's a piston single, and a business jet. Find the jet, since it goes faster and you'll rack up miles faster. Fly due south so that you don't fly over the restricted airspace to the north and get shot down. When you max out your flying skill, the game will give you a pilot's license. Personally, I don't think the game should do that, but it does; and it's not a cheat since you did it by playing the game as it's meant to be played. What might be considered a cheat is if you start flying, and then put a strip of Scotch tape on your Xbox controller to hold the right trigger button in and let your Xbox run all night long. By morning, you'll be a flying ace with a pilot's license!

The correct way to get a pilot's license is to pass flying school. You will unlock flying school soon after you leave San Fierro.

get 30% in flight school

THE ANSWER IS FUCK!

Don't bother wasting time trying to steal a plane at an airport & flying for ages - At Verdant Meadows Airstrip flight school I found that using the slowplay cheat code makes it so much easier to maneuver the plane. I did it in about 1/2 dozen tries compared to about 50 in normal play!