As has been posted here before, knowing the rules of the road is huge. Following distances, right-of-way, merging, basic green,blue,brown signage, etc etc.
Also instilling that driving the vehicle is the first priority. Not the phone, friends, radio, eating or anything else. Their lives are on the line and so are other drivers.
Also educate them about the aftermath of accidents beside dealth or dismemberment. Things like lawsuits, garnishments and true lifelong effects of bad driving.