You need to understand the reader. Just saying "serial" doesn't help much and makes you out to be lazy or evasive.
You have to understand the data rate, stop and start bits, parity or not. I would suggest you either go back to the "guy you got it from" or find the model number/manufacturer and google for that. If it's a commercial unit, there WILL be some sort of information about it.
You've probably spent more time asking for help than it would take to actually go and find it yourself.
One clue for you, though - the format of the card will probably not have it fields align on byte boundaries. You will need to take the received bytes, break them down into bits and look for the fields that way. often times different cards use the bits differently. I had to do this with an HID reader I used in a project.