Arduino / GSM module

used a USV SIM800-V3.9.3 GSM module with an arduino uno for a car anti theft security circuit. I used a power supply of 12v ,1.25A to power the GSM module. I connected the arduino Dtx to the GSM module rx, and the arduino Drx to the GSM module tx. I used my laptop to power my arduino while testing the program. But the GSM module is not communicating with the arduino. Please help me out. It's my final year project and am stock.