Get out before the car goes under.
Open the window quickly, if you can't get out fast enough. The idea here is to get the pressure inside the car equal to the pressure outside the car. Water exerts greater pressure than air, and if you are in a car full of air, you'll never get a door open. You won't be able to get the door open easily, but you can likely climb out the window.
Break the glass if you can't open the window - kick or smash a heavy object against it.
Recognize that whichever end of the car the engine is in will sink faster. The depth and current of the water may cause the car to flip over on its roof.
Leave your valuables. They'll probably still be there when your car is recovered.