A list I've comprised from a lot of research and no I'm not paranoid...
  1. Realize you are falling
    This is the first step. The faster you realize you are not on land the more time you have to complete the following steps. Ideally, you would have fallen off a very high bridge or structure because you know, more air time.
  2. Begin to open any type of window/door
    I'm not to sure about the door part, but as you are falling you could begin to roll down the windows- or as a precaution keep your sunroof open whenever you cross over bodies of water.
  3. Embrace the crash/splash
    It's go time.
  4. Get out
    The car is going to be rapidly filling with water so try your hardest to get out, but you may have to wait until the car is completely filled...that's when it reaches equilibrium so you should be able to swim out then. (Fun fact: you can not open car doors until your car reaches equilibrium. Some reports would advise you to hold you breath and just wait until then. But my lung capacity is brief and who knows how deep this lake is)
  5. If you left anything valuable behind swim back and get that
    I'm not talking about iPhones and wallets, I'm thinking more like kids or babies. Actually get them out first.
  6. Swim up the the top and wait for help
    The chances are high that someone saw you fall off and has already called the cops. If not then you're going to have to freestyle to land.
  7. Go to the store and purchase one of these
    0c404f58 c0fc 4a24 a2e0 df8f3b1dfc45
    That way you can break the window in case this happens again