the CEL codes list?

My CEL has been on for a couple of days, and I just had it read- P0304 which the AutoZone employee said was a misfire on cylinder 4.

I'm a automotive newbie, where should I begin looking for a problem? Is a misfire a one time occurance? Or a repeat thing? Should I just have them clear the code, or do I have a problem? Check the plug?

Any suggestions? Thanks in advance.

MUST SELL - ECM computer new for a 98 V6 mtx + pats Now trolling in a Red Nissan 4 x 4