How do I detect a specific object in a photo to trace it out?

Lets say I have this pic here:

and I want to seperate the car from the background to do further processing on it (like changing it's color or trace where the doors or windows are) what is the procedure to do so?

I've seen many tutorials on different examples but without explanation to the actual thought process behind it, and it looks like no solution fits all situations,can someone explain? any good recourses that can help me in learning image processing?