Start with your picture, probably one that is full of reflections, it makes it look better in the end.
Make any modifications that you want to in the image right now. It's a best bet to shave the door handles and the emblems, because they are a pain to "cartoonify."
Make sure you select the line tool and set it at black and start outlining the car, the panels, and the major reflections. Like the picture below. You can add the lines on a different layer if you'd like, but I did not choose to...
Then select the Polygonal Lasso Tool and select an area between the black lines that you made. Or select an area with a similar color, not necessarily one that is between black lines. Like so...
Select the eyedropper tool as illustrated above, and sample a color within that area of your selection and fill it with the paintbucket. If you do not know where the paintbucket is, it is under the gradient tool, just right click and it should reveal the paintbucket tool. Alternatively, you can press ALT+Backspace, and it'll fill it just fine
Just repeat this to the other parts of the car. Here's a look at the "cartooning" in process. You might want to make the windows black, it'll save you a lot of time. Also I used a gradient for the side windows because the original windows looked similar to that. Make sure, everynow and then zoom out and make sure everything looks right.
Keep in mind if a section that you filled with color looks like crap, then leave it alone. For example lights such as the headlights should be generally left alone unless you have some extreme patience. It's amazing how one section of the picture can make the car look like crap.
Here's the final result. When you save as a jpeg, go to File>Save for Web and put the quality at whatever you want, preferably the highest, and set the blur to 0.5, it'll produce a nicer result without as many jagged edges.