why is it so?

Hello All,
I am unable to understand when I execute etc.php,
it give me the error
"Warning: session_start(): Cannot send session cache limiter - headers
already sent ....."

can you solve this leral ?

TIA

--
Nayyar Ahmad

Lecturer
Faculty Of Computer Science,
Institute Of Management Sciences,
Hayat Abad Peshawar , Pakistan.
Office : 92-091-9217404 , 9217452
Cell     :  92-0333-9139461