This mission is started by Michael and begins at the FIB building. Once all is said and done, walk to Dave's car and get inside. It appears that they need some extra help for this mission, and they want to get Franklin and Trevor. Meet up at the spot marked on your map and watch the scene that follows.
When you regain control, you'll be using Trevor, who is flying the chopper. It controls similar to previous games, if you've played them, but just follow the controls on the screen if you haven't. Get to the building and when you get to the right position, Michael will get into position to start rappeling down it.
Follow the instructions on rappeling and keep going down until you see some Agency people interrogating your target. When you get the prompt to smash through the window, do so and then switch to Franklin. Shoot the agents, who are red dots on your radar. Once you've shot enough, switch to Michael and kill the rest.
Watch the scene and you'll go back to Franklin, who has a chopper heading to his position. Shoot the pilot to make it drop quickly, then switch back to Michael, who should now be equipped with a machine gun. You want to shoot at the helicopters chasing you until they either catch on fire or you hit the pilot, which might prove difficult with your current weapon.
There will be a total of three choppers on your tail, so try to concentrate on one until they go down, then you can move to the others. Once all three choppers are out of the picture, switch to Trevor and fly the helicopter to the parking lot, landing it in the lot when you get there. The mission will end after the cutscene.