Spark plugs are an important aspect of every vehicle, as it's these components that ignite the fuel mixture in a car. In the past, a car could literally not run if the spark plugs were faulty. Nowadays, electric ignition systems make vehicles a bit more resistant to a few spark plugs not working properly, and drivers can go longer between needing to change them. That said, fouled spark plugs can affect your performance, so it certainly never hurts to consider replacing them.
It's expected that plugs will fail over time due to the electrode being eaten away by combustion inside the engine. However, there are many other problems with the car that can cause a spark plug to fail, so they actually serve as a good diagnostic tool for other parts of your car.
For example, you might notice a buildup of brown or black gunk on the top of your spark plugs. This is typically oil that has escaped from the engine chamber, and depending on how much is there, you may have a leak. Gasoline on the fuel sparks or a carbon build up may indicate that your car is "running rich," which means it's getting a much higher amount of fuel than necessary. This could be a problem with the car's fuel injection settings or sensors. It could also be related to any modifications you have made to the engine on your own.
Replacing spark plugs is one of those auto maintenance jobs that many drivers can likely handle on their own, as it's not all that complex and requires very little knowledge of the engine to do so. If you do opt to change your own spark plugs, make sure you let any mechanics who give your car a tune-up that the spark plugs were recently replaced.
A set of spark plugs should last for at least 1-2 years if not much longer, so it's pointless to replace them yourself if you accidentally let an auto maintenance worker do it later on. Many include spark plug replacement as part of a normal tune-up, so be sure you are clear that your car doesn't need new plugs if they were replaced recently.
To replace a plug, simply disconnect it from the wire, loosen it with a ratchet, then attach the new one and tighten with a wrench. Be sure to reconnect the plugs to the correct wires and attach the wires in the same order that you unplugged them.