2005 Chevy Cobalt engine runs rough at idle

My 2005 cobalt runs very rough at idle. I had a code that idicated replace the knock sensor. Replaced this. Now I am changing the spark plugs and wires. The next thing that was suggested by an Auto Zone employee is changing the air filter and the fuel filter. I checked the air filter and it seems to be pretty clean. COuld my problem be it just needs a tune up. Just pulled one plug and the gap is way off and the plug is cooked. In your opinion, could this be the problem? I am a 68 year old single woman trying to repair my own car (picture that)