When your car runs rough, it's usually something simple. You can check the most obvious things without taking your car to a mechanic. When your engine misfires, it doesn't have to be a mystery. You can check it yourself without a tool chest full of tools. Once you find the problem, you can probably fix it yourself.
Open the bonnet and start the car. Listen to the engine for a few minutes to establish a sound in the pattern of the misfire.
Pull the rubber cap off the back spark plug. Listen to the engine. If the misfire is noticeably worse, or the engine dies, put the cap back on the spark plug.
Pull the cap off the next plug in sequence, listen and then put the cap back on the plug. Do all the wires in sequence. When you get to the cylinder that is missing, there will be no change in the sound of the engine. Pull it off and put it back on several times to make sure you have the right cylinder. Leave it loose and shut off the engine.
Remove the spark plug with a spark plug socket and ratchet. Replace the spark plug with a new one. Put the spark plug wire back on and start the engine. If the engine continues to misfire, shut off the engine. Pull the spark plug wire off the engine and discard it. Replace the spark plug wire with a new one.
Start the engine. If the engine continues to misfire, the problem probably lies with the fuel delivery system.
Take your car to a mechanic if you can't fix the misfire with spark plugs and spark plug wires. It could be a bad fuel injector, or a problem with the car's computer. The engine could also have a bad valve, or a hole in the cylinder -- not something you can fix at home.
Don't grab a spark plug wire that is frayed or burnt. It could shock you.