Brace yourself from the impact and quickly undo the seat belt. It is because, if water gets through it, it will get jammed and it will become a lot more difficult to escape in such situation.
Open the window as soon as you hit the water. The electrical system of the car will work for few seconds in the water, so try opening it quickly. If you are unable to do it, you'll have to break the window
If the door is jammed because of the pressure, you'll have to break the window. Use a small hammer from the toolkit to break the window. Water will be rushing inside, once the window is broken, so expect the water and use your strength to rush out of the window
If the car is equalized and the entire cabin is filled with water, you must move out of the car quickly to ensure your survival.
Push off the car and swim to the surface. Swim towards the light if you cannot track the shore. Next, you can rush to the nearest hospital and get your entire body checked.