Olle
This actually caught me out first when I tried for first time to connect it all.
The Pins 7 & 8 are RX and TX on the Arduino Board. The RX and TX on the GPRS/GSM Sheild I used are Pins 0 & 1.
So Pin 7 from Arduino goes to PIN 0 on GSM Sheild
Pin 8 from Arduino goes to PIN 1 on GSM Sheild
For Mine to work the Jumpers on GSM sheild are set to Softserial
And of course connect the GND to GND for common GND.
In the Code Make sure
#include <SoftwareSerial.h>
SoftwareSerial SIM900(7, 8 ) ; // tells Arduino what pins to use for RX and TX
Regards
Ian