in general you need to keep your track in to a basic idea.
what would be deferent from the time that someone come near or even touch the door
examples ( light, sound, temperature ect. )
after that you can creat in input point that continuisly check this value and at a change it trigers your output pin.
a door knob made from clear plexiglass dome with lightsensors in side would be my choise.
i am preaty sure that you can find everything easy.
for your first experiment at least while Paul is right, his idea is more apropriate.
good luck with your project.