Inquire at the ground transportation desk, they will find the correct service for your destination. As with many U.S. cities, several hotels provide shuttle service to and from the airport. Check with your hotel when booking, or after you arrive. All major rental car companies, and many smaller ones, are represented at the airport and throughout the city. Los Angeles is very sprawling, so if you are going to more than one area it would be advisable to rent a car.
Car rentals are cheap compared to taxi or mini-bus fare.
All of these airports are served by either Airport Coach, or Supershuttle. If you are going to downtown LA, it is wise to take one of these services. There are phones and information booths in the baggage claim areas of the airports where you can book service. Taxi service is also available at all airports. If you are going to an area near the airport, the taxi is the easiest way to go.