GSM Receiving Sms

Hi guys i badly need help, about receiving text message i cant find code that works i only found code that can send sms and it works. Here is the code:

void send_msg(char *number, char *msg)
{
  char at_cmgs_cmd[30] = {'\0'};
  char msg1[160] = {'\0'};
  char ctl_z = 0x1A;

  sprintf(msg1, "%s%c", msg, ctl_z);
  sprintf(at_cmgs_cmd, "AT+CMGS=\"%s\"\r\n",number);
  
  sendGSM(at_cmgs_cmd);
  delay(100);
  delay(100);
  delay(100);
  sendGSM(msg1);
  delay(100);
}

void sendGSM(char *string){
  Serial.write(string);
  delay(90);
}

to send sms: send_msg("number", "message");

can you help me on how should i code to receive message.

i am using Arduino mega 2560 and gsm shield (http://www.e-gizmo.com/KIT/gsm%20shield.html).

the sample code from the arduino page dont work it only display "SMS Receiver" on the serial monitor. Hope that someone can help me. :)

Anybody? :)