Strange characters after typing an AT command in serial monitor
for those who have this issue, i resolve this by changing the speed baud of my GPRS Shield :

GPRS.begin(2400); 
Serial.begin(9600);


and if you want to test AT command directly on the serial monitor you must use 2400 anywhere 

GPRS.begin(2400); 
    Serial.begin(2400);


and don't forget to change also  the speed on the serial monitor to 2400
I hope this will help someone