How to fix my cars check engine light?

I have a 1998 Ford Mustang 3.8 leiter 6 cylinder car. My tag is expired but my check engine light is on and I don't know how to turn it off. The previous owner said that she use to turn it off by unplugging the battery and that it would reset the check engine light. During the process of doing this the first time we accidentally left the metal touching the battery still and now my head lights nor my brights will turn on. The check engine light went off but turned right back on when the car was restarted and did this the second time we tried as well. When it was tested at auto zone they said I needed an Oxygen Sensor, Camshaft Sensor, EGR Valve and an EGR Valve Sensor when I called my dad he said only the Oxygen Sensor had anything to do with the check engine light. My question is what did I do to my head lights? What do these senors do and are they majorly important? && how can I cut off the check engine light long enough to get emissions? Please don't be mean I know I'm bad with cars & thanks for all the help.

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