#include <SoftwareSerial.h>

SoftwareSerial SIM900(10, 11); // (RX, TX). please zoom image above to see exactly what is going where.

void setup()
{
  SIM900.begin(19200);               // the SIM900 baud rate
  Serial.begin(19200);
}


void loop()
{
  if (SIM900.available())              // if date is comming from softwareserial port ==> data is comming from gprs shield
  Serial.write(SIM900.read());
  if (Serial.available())                // if data is available on hardwareserial port ==> data is comming from PC or notebook
  SIM900.write(Serial.read());
}