There are many ways i'm sure that will be offered up here but here is mine. First open your image in photoshop. Then select Edit In Quick Mask mode as highlighted below.
After you select quick mask mode choose the brush tool then hit D on your keyboard to change the brush to black and paint around the car with black leaving only the car visible. It will appear red while in quick mask mode. Once you have carefully done this return to normal edit mode. You should now have a nice selection around the car. Hit ctrl-c to copy your selection then paste via a new layer. You should now only have the car in the new layer minus the background. Open up the image you'd like as the background and copy and paste it into your car photo. Voila, hope that made sense.