Instructions
Play through the story of "Grand Theft Auto IV" until you unlock Happiness Island. You can't steal a helicopter until this island is unlocked through the natural progression of the game. To unlock Happiness Island, complete the bank robbery mission on the second island, right around the 30 percent completion mark.
Head to the east end of Happiness Island and locate the "Helicopter Tours" building.
Jump into an empty helicopter. You won't need to sneak around much because the helicopters in this area are unguarded. Hit the appropriate hijack button for your console: "Y" on the Xbox 360 and "Triangle" on the PlayStation 3.
Control the helicopter using a combination of the trigger buttons and analog sticks on your controller. The right and left triggers control your altitude, while the left analog stick controls your direction. To move forward, tilt the analog stick up and hold the right trigger. To land the helicopter, hold the left trigger until you are safely on the ground.