Hey Pradeep! It really depends on your relay, however, I'd assume that you could simply use a transistor of some sort (a MOSFET would be the likely, simple choice). The 8051 output would go to the gate of the FET, which then controls the current to the relay switch. Finding a FET that is tolerant of over 12V would be easy as well. As long as your microcontroller and 12V source have common grounds, that should work with just the FET.
You need to make sure that you get a logic level MOSFET that will turn on when at whatever voltage your 8051 puts out. These are increasing common but not MOSFETS will switch with 5V less the output drop, or 3.3v, if that's what you're using. 8051outputs may not have much pull up so a pull up resistor, say 5k from the 8051 output to Vcc may help the switching,
I think that you saw it and noted it's presence will suffice. [In other words, I've got a busy day and even taking the time to do that diagram was an impact ;]
