Batteries will often go dead at the most inappropriate times. This is where jumper cables come into play--you can often spark your car battery into life when it is dead by connecting jumper cables to your car from another car that is already running. It is important to be careful when connecting these cables, however, to make sure that you do not cause any damage to the cars or to yourself.
Clean any areas of the jumper cable connectors that show signs of corrosion. Mix some baking soda and water together in a bowl to form a paste. Use a towel with a little bit of paste to wipe off the jumper cables, then dry them with another towel. This will stop a voltage drop from taking place, which will not allow you to properly start your vehicle. Ensure that the cables are fully intact and are safe to use.
Check to make sure that both batteries are of the same voltage. The voltage of a battery will usually be listed somewhere on the battery itself. If there is no such listing, use a voltmeter: remove the battery's negative wire with a wrench (grasp the clasp with the wrench and turn), then remove the positive wire the same way. Take the voltmeter and place the red clamp on the positive end of the battery and the black clamp on the negative end.
Park the two cars close enough to each other to connect the jumper cables. Make sure that the cars are not touching, however, as this could be dangerous. Put both cars into "neutral" gear and leave the parking brake on for both vehicles.
Connect the red cable to the positive (+) post on the dead battery. Then connect the red cable to the positive (+) post on the operating battery. Make sure that you do it in this order to avoid potential damage.
Connect the black cable to the negative (-) post of the good battery. Connect the black cable to the negative (-) post of the dead battery. It must be done in precisely this way to avoid damaging the batteries on either of the vehicles.
Attempt to start the car with the dead battery. If the battery turns over, allow the car to idle for a few minutes with the batteries still hooked to each other. When the time comes to disconnect the jumper cables, unhook them in the reverse order that you put them on: negative for the bad battery, negative for the good, positive for the good and positive for the bad battery.
Drive the car with the bad battery to a vehicle repair shop that can assess the state of your battery and charge it up or replace it if needed.
