Accept the taxi role at the job center and then go to postal 408 near the Casino. In the parking lot of the Downtown Cab Co., there will be a device on the wall that you will be able to clock in at. Press E when prompted to clock in.
Once Clocked in, you can either head to crab cab keys next to the the grey door olong the parking lot wall. You may choose to grab a taxi vehicle from the garage, or you may use your own vehicle.
Once Clocked in and inside a vehicle, you can press your Job Menu “F6” to open up. You can drive locals around for a fee deposited into your bank account for every drop off, or you can taxi other players and use the “billing” Option to invoice them.