how to interface arduino and gsm sim900

Using Arduino Project Guidance
1

Help me to interface with gsm module

2

Buy the shield, plug it in.

3

http://tronixstuff.com/2014/01/08/tutorial-arduino-and-sim900-gsm-modules/
http://www.seeedstudio.com/wiki/GPRS_Shield_V1.0

feed sim900 no more than 4V. Read the manual