Microcontroller › AVR › how to control room lights via pc and microcontroller › link for serial interfacing
link for serial interfacing in AVR
Atmega8 interfacing with relay ckt
when you send logic low at PC0 the LED will be OFF and when you send logic high at PC the relay switch and the LED will glow. In your case instead of LED you can connect bulb/ tube light/ fan , etc.
nice project all the best