first you need to find a sensor circuit you want whatever it is..
understand the circuit how it works. it should have a digital output that will trigger the input of your parallel port for the program to know that the sensor senses something unusual.
example your have smoke detector circuit… when the sensor does not sense any smoke at all the output of the smoke sensor circuit is 0 or logic low and if smoke is sensed then it will trigger to logic high og 1.. thats all.. actually you need only 2 wires to do that exluding the power lines.