You're wrong on the SES light, it indicates a variety of issues beyond emissions and a cap. You're also all over the place on your guesses. Nothing wrong with admitting to your self with car technology today, you're in over your head.
Autzone will read the code with a generalization of what your problem is, but they are unable to diagnose what the actual malfunction is. They will offer to sell you parts for every possibility. But to repair what is not working, you need to go to a capable technician