There are two ways of connecting GSM module to arduino.
1. you may connect the Tx pin of GSM module to Rx pin of Arduino and Rx pin of GSM module to Tx pin of Arduino. (while programming Arduino you have to disconnect wiring in Rx and Tx each time you burn the program to arduino.)
2. Use other digital pin in order to remove above difficulties say you are going to use pin D9 and D10 for Rx and TX respectively, then you have to define pin for Tx and RX say SoftwareSerial Sim900(9, 10); // RX, TX and set the communication baud rate of GSM Module Sim900.begin(9600).