This might sound annoyingly obvious (and not helpful) but the easiest way to control the lights and appliances is to switch them off at the wall when you leave the room or put up a sign saying 'Please switch off lights / appliances when not in use'!
There already systems like this called mood lighting which control lights / blinds etc. They can be controlled from a central control interface or connect to a PC so more complicated things can be done.
If you want to go the 'intelligent' way you could have some sensors in the room / house which sense when there are people. PIR, motion sensors, break the beam sensors etc, then some intelligence like a uC which eventually drives transistors / relays to control the I'm assuming higher voltage appliances.