Roll down the windows as soon as the car hits the water. This will allow water to pour into the car and equalize the inner and outer pressure on the door. This is your most likely escape route.
Break out windows that will not open all the way with a strong kick or a heavy object such as an emergency hammer, tire iron or anti-theft device. Shield your exposed skin and eyes from the glass shards when the window breaks to avoid injury.
Release your seatbelt immediately and climb out the front window. If your seatbelt is jammed, use a knife, glass shard wrapped in fabric or anything else you have handy to cut the seat belt. Push children out ahead of you.
Wait until the car is nearly filled with water if you are unable to climb through the window. When the water is nearly to your head, take as deep a breath as possible, hold it, and use your legs to push the door open. Swim away from the car and to the surface.