You don't know the force without further information. If you know the acceleration, you can use F=ma, m being the mass of the car. But the acceleration is (probably) changing as the car comes to a stop, so you don't know the acceleration from the stopping distance, which is probably the data you have or can estimate.
You can get an order of magnitude by assuming constant acceleration, so force times stopping distance = work done = kinetic energy before impact = (1/2)mv^2.
Post again in this thread if you need more help.