Vehicle Requirements: Since Grand Theft Auto IV, Prostitutes have had specific requirements for cars that they can do their "magic" in. They cannot be picked up in the following vehicles:
- Emergency Vehicles
- Bikes (for obvious reasons)
Some vehicles are restricted due to size, as it can be heard that "I bet you're flexible, but not in here," or "I think this car is a little too small for that..."
Also, some prostitutes are called Luxury or Popular prostitutes, and you need a flashy, expensive car to pick them up.
Just basically use a generic civilian car.
Time Requirements: Prostitutes are only findable at night, as it seems like it is only then when they show their true colors.
Cost: Prostitutes are very cheap, with 3 options for services, costing you $50, $70, or $100. All 3 have different lengths and descriptions.
Note: You can kill them afterwards to get your money back.
Make sure you take them to a secluded place, be generous, make sure you use protection, and good luck with the ladies!