having problem with my check engine light

i have a 99 mercury cougar ive had for about a year n a half i had a problem with my check engine light like 2 days after i bought the car and got it checked and they said it was my oxygen sensor so i went and got it fix but after i got it fixed the light came on again for like about 1 to 3 hours,then it went back off, so now its happened again a year later and auto zone is telling me that its my catalytic converter because a code po420 keeps popping up and it says something about cylinder 1 isnt performing correctly and something about a OBDII , i just wanted to find out if this is the problem or do i just need a tune up, cause i havnet got a tune up on this car yet ever since i bought it because i just havent had the money for it , i bought some spark plugs the other day for it and i was gonna see if that works, soooo somebody please tell me what should i do , cause im really having a hard time finding a catalytic converter for this car , theres none avalible in the local stores :disappoin

Are you only getting one code or multiple codes? If multiple codes then post all the codes. Cylinder not performing correctly is vague and can mean a lot of things.
OBDII is the onboard diagnostic computer.

P0420 - Catalyst System Efficiency Below Threshold (Bank 1)

A number of things can cause this. What exactly is it saying about cylinder 1? If it is a misfire it could actually be causing the p0420 code.

that first line you put up is what the code is saying , im hoping i just need to change my spark plugs because i havent changed them since i had the car.do you think i nee to change the wires and the distributor cap and rotor too, or should i check the oxygen sensors , i herd there's two o2 sensors in my car , is this true?

Doing a tune-up won't hurt. The following is two links you may find helpful.
http://www.autozone.com/servlet/UiBroker?ForwardPage=/az/cds/en_us/0900823d/80/1b/bd/bb/0900823d801bbdbb.jsp
http://www.dansmc.com/Spark_Plugs/Spark_Plugs_catalog.html

that first line you put up is what the code is saying , im hoping i just need to change my spark plugs because i havent changed them since i had the car.do you think i nee to change the wires and the distributor cap and rotor too, or should i check the oxygen sensors , i herd there's two o2 sensors in my car , is this true?

I would go ahead and do the plugs and wires because like KimMG said it wouldn't hurt. Oh BTW you don't have a distributor and rotor.

I would really recommend checking out http://newcougar.org/forums/index.php