Is there any difference between analog ground and digital ground?? If yes what? if no why there are two grounds??
How can i decide which one to use??
I am asking about ATmega328P.
With what chip ?
The ATmega328P ? or with a radio chip ?
Normally both grounds are connected to the ground layer of a pcb board.
The analog power supply (The 5V or 3.3V) is sometimes filtered to reduce ADC noise.
However, there is more to it.
I know that TI has written (difficult to understand) documents about it, but I can't find it right now.
Wow, 22 million hits. That's more than my pages get. 
Try this article.
I have a link to a very good TI article also somewhere, not sure where I saved i.
Ground Plane - Lord_of_the_board.pdf (183 KB)
Often on designs using analog I/O there is a separate ground used for the analog signals that does tie to the main system ground. The purpose is to prevent electrical current from flowing in the analog ground path and corrupting (adding noise) to the analog signals being brought back to or going out of the system. I have worked with Plasma cutting and welding systems where this was very important because noise in the analog circuits could affect machine motion and produce quality problems with welds.