That spark - that's what makes your engine go. It ignites gas-filled air in your engine, setting it alight and pushing pistons, dozens of times a second, thousands of times a minute. Those little spark plugs are putting a lot of work into your engine. It's only fair to give them a bit of your attention.
In fact, you should probably check them every 10-20,000 miles. They'll usually last longer than that, but it's a good idea to keep an eye on them. Or if your engine isn't running smoothly, give them a look. It won't hurt, and it may well help.
First Things First
Just to make sure you don't put yourself through a lot of hassle, remember these two things.
--Let your engine cool. Seriously, don't burn your hands.
--Change the plugs one at a time. Otherwise, you could get wires mixed or crossed and then your plugs will spark at the wrong time and it's just a huge mess.
To remove your spark plug, you need a ratchet wrench with an extension, as well as a socket that fits over the spark plug. There are sockets on the market that are made for removing spark plugs - basically, they have a rubber ring to hold the plug and bring it out, to avoid needing to remove it by magnet-on-a-stick or some similar method. (Remember to just remove one at a time.)
The spark plug will be under the ignition wire, which should pull off reasonably easily. Just remember not to force it or you could end up with an expensive breakage. Once the wire and the boot (the part that attaches the wire to the plug) are out of the way, unscrew the plug as you would any normal screw and take it out.
The condition of your spark plug when you remove it can tell you a lot about the operating conditions of your engine. Check the link to see what each means.
If you're just checking your plug, check the gap between the ground and center electrodes. The size the gap should be is under your hood. Most of the time, new spark plugs will already be gapped properly. You can also clean off the plug with a wire brush.
If you're putting in a new one, make sure your old and new spark plugs look the same. If not, something's wrong and you should probably re-check what you're putting in your engine.
To replace, put the plug in the ratchet extension, put it back in its outlet, and hand-tighten it, then turn it just a bit more with the ratchet wrench. Replace the wire, and you're done!
A good spark plug keeps your engine fuel-efficient and purring like a baby jaguar. So keep them clean and ready and your car will run happily.