Dead battery? Bad starter? Bad/dirty connections on battery cables? If you know the battery is good, you have to listen when the key is turned. if there is an audible "click" when you turn the key its mostlikely the starter.
If your car battery is dead, you will not be able to start your vehicle without a jump using jumper cables. When the car won't start and the lights on the dash aren't working, it is most likely your battery.