Compensation after damaging my car

I was driving the car down the A4 motorway. I wantred to take the exit and  after changing the driving lane from 3rd into 4th, which was available at that moment, my car was hit from the back on it's right side, then it was grinded for a while on it's right side by the yellow truck. I lost the control over the vehicle.

In fact I was really lucky because I was driving my little child. We are ok but the car is toltally damaged. In fact I have never been to this kind of situation and only now I realised that I should get some compensation.. Could you explain me step by stape what to do?