Why does my 2002 hyundai elentra misfire?

Is it misfiring on just one cylinder (code 0301, 0302, 0303, 0304 etc.) or multiple/random (code 0300) cylinders?
It takes all 4 ingredients to fire a cylinder - Air, Fuel, Spark, and Timing. If you only have one misfire code (P0302 - Cylinder #2 Misfire), it should be a relatively easy diagnosis and fix. Let's start with Spark. I'm going to assume you have a code reader. First, remove the spark plug in #2 cylinder. Examine the threaded end. Is it wet and/or smell like fuel? If so, chances are the misfire is caused by a weak/absent spark. To confirm, swap it with a known good cylinder - example #3 cylinder. Clear the code. Start it and let it run for a couple of minutes. If a code appears with P0303 (the last digit refers to cylinder number), then you have found the culprit - bad spark plug. If the code reappeared with P0302 (Cylinder #2 again), then swap out the spark plug wire (both ends). Clear the code. Start and run for a couple of minutes. Again, if it appears on Cylinder #3 - the plug wire is the culprit. If the P0302 code comes back, swap the coil. See where this is going?
Now, when you first pulled the plug from Cylinder #2, if it wasn't wet or smelled of fuel, chances are Spark is not the issue. Was the plug dry and/or ashy? Possible cause is no fuel reaching the combustion chamber (cylinder). Using an automotive stethoscope, listen to the fuel injector for cylinder #2 while the motor is running. Should hear a regular and rhythmic ticking sound. Like a fast clock. (Use a long handled screwdriver touching one end to your ear and the other to the top of the injector if you dont have a stethoscope). No clicking or irregular clicking means fuel injector is not working (no sound) or is not working properly. Like searching for the Spark problem, swap the injector in #2 cylinder with #3. Clear code. Start and run. If the problem moved to #3, ithat injector is the culprit. If it stayed on #2, it is likely your wiring to that injector - check the whole harness for loose connections or swelled with oil wires/connectors. Keep pecking away at the fuel delivery system for that cylinder. Stay looking for things that affect only that cylinder.
When you pulled the plug, was it fairly normal looking? Then this misfire code might only be a timing problem. It is likely not a timing problem in your case, because timing affects all cylinders, but I thought I would just mention it since Timing is an important part of combustion. Also, if timing were a culprit here, another code would usually appear signifying the Camshaft Positon Sensor or Crankshaft Position Sensor was/is failed, or it is possible for the timing belt/chain to skip a knurl on a sproket and throw the timing off, but like I said, it would almost always cause multiple/random (Code P0300) misfires.
And finally, when you pulled the plug earlier, if it was an oily sticky mess, then the culprit might be Air - or compression/lack of compression. Lets hope not, because when Air is the cause of a misfire, it is usually an expensive fix. Need to perform a compression test. If you have little or no compression in the cylinder, it is likely a bad head gasket or a stuck/sticky/broken/chipped valve (exhaust or intake). Any of these things are bad news and expensive to fix in a shop. Also bad news and time consuming and difficult task for a shade tree mechanic.
Now, clear that code and get started! Good luck!