Becoming a taxi driver is one of the easiest and fastest ways to make some quick cash in Los Santos while staying on the right side of the law. The city's premier taxi business is Downtown Cab Company which operates out of its main depot on the corner of Tangerine Street and Mirror Park Boulevard in East Vinewood. Open your map and click on the briefcast mission icon in the northeast corner of the city to find your way.
Find your way to the taxi depot and look for a spinning question mark near the door. When you get close enough, you will be prompted to press a button which will start the mission. You must have a taxi in order to do this mission. If you can't provide your own, you may rent one at a cost of $100 per day. This a non-refundable rental fee, but you may keep the taxi for the entire day (until it despawns or the server restarts).
Once the mission begins, simply drive around populated areas until someone calls for a taxi. This usually takes between 30 and 90 seconds. Once a call is received, drive to the marker as indicated on the minimap and stop the taxi near the customer. They will enter the vehicle and instruct you on where to go.
Drive the customer to their destinations as fast as possible to earn the maximum amount of money. However, be mindful of traffic laws and damage to your vehicle. If you get pulled over by the police or damage your vehicle too much, it will take even longer to arrive and your passenger will not be happy.
Once you've dropped off your customer, simply resume driving around until the next customer calls. You can abandon the mission at any point by leaving the vehicle for a little while.