Shahzad,

What I got from your message is that you wish to control the 12V supply of some device through microcontroller.

Generally, it is advisable to disconnect the VCC of such devices rather than GND. This leaves a device with GND connected, but VCC dis-connected.
In order to disconnect the 12V from your device, you need a PNP transistor with at least 50V VCE rating and at least double the current rating (double of your load current).

Now connect the transistor as follows:
1) Connect emitter to +12V
2) Connect collector to the 12V supply point of the load.
3) connect a 100K resistor from emitter to base. This will act as a pull-up for base and will keep the load in off condition if the base is left open.
4) Now connect a resistor of some suitable value(depending upon the base drive for PNP which futher depends upon your load and hfe of the PNP ) say 2.2K to the base of PNP.
5) The second end of this base resistor (other is at the base of PNP) is the point to turn on and off your load.

If you hold this point at 12V, your load will be off. If you apply approx 11.3V at this ponit, the transistor will trun on and supply power to your load.

Now there is a problem. Your micro has 5V supply, thus it can not make this transistor turn off.(since below 11.3V, the PNP will always be on, regardless of the status of the port pin which is controlling the PNP)

Now there is a simple solution to this problem. You can use an NPN to control the base of PNP. Do as follows:

1) Connect the free end of 2.2K resistor to the collector of NPN.
2) Connect the emitter of this NPN to GND
3) Connect the base of this NPN through 4.7K to a free port pin.

Now when you set the pin to logic 1, your load will turn on, and clearing the bit will turn off the load.

Sorry, I don't know how to post images.

Regards

Ijaz Ahmed