Just about everyone who's owned a car has been there - you turn the key in the ignition, but the engine doesn't start. A car that doesn't start can be a frustrating problem, but fortunately it's one most amateur mechanics can handle.
Even those who can't tell a radiator from the exhaust likely know where the battery is on their car and may even have an idea of how to jump it. A dead battery is by far the most common reason for a car not starting. Typically, this problem will be accompanied by a clicking noise when the driver turns the ignition. If the car is completely silent when you turn the key, it may not be the fault of the battery, but the cables attached to the battery. The connections and the post can become corroded over time and not transmit electricity to the battery, in which case you'll need to clean the parts or get new cables after you jump the battery.
Before you delve into this issue, however, you'll need to jump your battery. This process is easy and it's worth doing before you start assuming other parts are at fault. Take out your jumper cables and park your car close to the vehicle that is who is giving you the jump. Open both hoods and attach the red clip of one cable to the positive terminal on the dead battery. Then, put the other red clip on the positive terminal of the good battery. Attach the black clip to the negative terminal on the good battery, then attach the other end to an unpainted metal surface on the dead car, preferably far away from the battery. Be sure you do not attach this clip to the negative terminal on the dead car, as many drivers are wont to do. Jumper cables can sometimes give off sparks, and sparks close to the battery can ignite gases and cause an explosion.
Wait a few minutes before you attempt to start the vehicle. In most cases, the car will start. However, this doesn't mean you're out of the woods yet. The alternator could be causing the problem. With the car in park, turn your headlights on, step on the gas and rev your accelerator. Have your friend who just helped you jump the car stand outside and watch the lights. If they dim or get brighter as you step on the gas, it's likely the alternator that's not providing enough juice to the battery. See an auto maintenance expert to get this straightened out.
If the car doesn't start, it's possible your battery is completely dead and you'll need to replace it. This usually isn't a major hassle, but you won't be able to start the car until you get a new battery. Be sure to reconnect the jumper cables and try again before you call a tow truck.
Of course, the issue could be unrelated to the electrical system. If you hear the engine begin to crank and then fail, it could have to do with the fuel system. You may be out of gas, or a fuel line could have ruptured that is preventing gas from reaching the engine (you should likely see and smell gasoline if this is the case). The spark plugs may also be at fault and could require replacing.
Sometimes, the engine may start and then die shortly afterwards. This is common on older vehicles with carburetors. Most new vehicles have fuel injection instead, in which case you'll need a repairman to take a look at this complex system.
Other drivers may have noticed their car only has trouble in certain types of weather, such as when it rains. This is likely related to the distributor cap, which has a tendency to trap moisture. Many auto parts stores will sell mechanic's solvent, which can be sprayed inside the distributor cap to quickly evaporate any liquid. It's not a bad idea to have a can of this in your garage - especially if you live in a rainy area.