As you are starting out wanting to draw and paint cars I would suggest you use a graph system on the photo to transfer to a graph on your chosen support. This will break down the car to small managable sections. How small depends on the size of graph you use. The smaller the graph the more accurate the detail and lines will be, the larger graph will result in a free line work.
As cars are very definite in their shape and form I would suggest you start by using a smaller squared graph to transfer what you see to your graphed support. Ensure that the scale of the graph is equal to the graphed photo or double/triple... the image on your support. In effect say for example 1 inch squares could be 1,2,3,4 inches on your support.
As you progress with painting cars your graph will become bigger with your subjects and may even not need to use one with practise and experience painting cars.
Hope this helps.