Once far enough into the game after you have stolen the Lost's plane as Trevor the option to purchase the Cab Co will be available to you, now select Franklin as your active character and travel to the upper right hand corner of Los Santos City in 'East Vinewood' where there will be a House icon with ($) inside it in this location.

Once the purchase of 200,000 is complete you will immediately get a call from Raul who introduces himself. Now you need to wait for a second call from Raul which may take some time so try to stick with Franklin and just play missions like normal, after 1-3 real hours you should get the phone call.

In the call Raul asks if you'd like to work on a 'Private Fare' to earn some extra money. Push the A to Accept. After this, you have on your Map a new green Symbol 'P'. Drive to them and pickup the Passenger.

If you need any help feel free to PM me.