How to pack a car emergency kit will depend on your location and driving habits. Before putting an item into your kit consider what kind of driving you do to determine if it makes sense. If you pack everything listed in this article you should have one of the most complete roadside emergency kits around.
Every car should have some essential items in a location easily accessible to the driver. Have some water (in a plastic container if you live in a cold environment so that it won't crack and split open if it freezes) and non-perishable food (chocolate, granola or fruit bars, nuts, etc.) in case you get stranded or just peckish, a first aid kit to treat any injuries and a window breaking tool and seat belt cutter to make it easier to escape the car if you need to get out quickly. Pack a fire extinguisher to put out any small fires, and a flashlight and extra batteries to help see if it's dark. Keep some spare change around for tolls, parking, and coffee, and a pencil or pen and paper for taking down notes or starting a fire.
There are other important items to keep with you that can be kept in the trunk. Have a few road flares or reflective triangles to warn oncoming traffic of your stopped vehicle. Get some jumper cables to help get your car running again in case the battery dies, a tire gauge so you can regularly check your tire's air pressures, and a tire pump so you can keep them properly inflated. Pack some tire sealant in case you can't change a flat. While you're at it check to make sure your spare tire is properly inflated and ensure that your jack works properly.