You first need to know the protocol to communicate with the HY502 module.You need to know the commands..
am attaching a datasheet along with this post, you can go through it.
First you can check the protocol by sending commands through a terminal which supports to communicate in hex mode
like 'Realterm' you can freely download it.
You should have a suitable RS232 to TTL converter inorder to interface it with PC.
make connections from reader to rs232 converter as per the J1 connector list mentioned in datasheet.
connect RX,TX,VCC,GND properly leave other pins .
send below mentioned command when card present at baud rate 19200(The red led glows and remains stable till card is present)
0x02 0x20 0x22
reply- AA BB 06 20 42 F9 BF 3D 1F (ID is in between 20 and 1F)
reply-AA BB 02 DF DD(if no card is present)
See the datasheet for more details......