More detailing. There is going to be some trim in the side of the car just below the door. We are going to create an indentation for the trim. Select the polys shown in red and inset them. Tweak vertices so that you don't get any dimpling on your surface and so that you can get the right shape.
Extrude the inset polygons inwards a little bit to create the indentation. Try to get a good shape like I have shown in the picture.
When you smooth the geometry you should get a nice groove as shown.
Next step is to flesh out the door so that it fits perfectly with the body of the car. Select the red edges shown and extrude them inwards a small distance. Adjust vertices to get the shape right.
Basically, the next tew steps are just extrudes like what we have been doing. What you have to do is follow the same extrudes that you did on the door area on the body of the car. Extrude the green edges in the direction of the red arrows. This picture shows the side of the door closer to the rear of the car.
The same operations shown from the other side of the door. Just make sure that you follow the shape of the door area on the body of the car that you did earlier so that the door fits perfectly in that space.