You must have a communication path. Most likely through you RS232 PC port.
Then you need a communication driver. Usually you use the PLC's own programming software. It could have the ability to do "on-line" monitoring.
You need tha cable to connect the PLC with the PC.
You could also use a monitoring software (a type of SCADA) which has this communication driver.(make sure they have this driver BEFORE you purchase)
You could also write your own in Visual Basic or other language.