what should i do to get valid rssi level please tell me in detail thank you.

I want to get different RSSI level at every feet apart from transmitter is it possible for xbee S2 if it is then what to do and how to do.

You are going to need to get something like a spectrum analyzer and place it in between the antenna port and antenna. You are also going to need to have an RF product that has less than 1mW of output power. Even then you are most likely not going to see changes at a 1 - 2ft distance.

you can check the rssi level at LEDs using simple circuit just like shown in development board.
circuit reference sheet 2 (upper left side circuit lebel as RSSI LED stack) of following SCH