use sim 300 module its easily available, you will require max 232 IC for interfacing the GSM module to the micro controller, AT commands are used for issuing commands to the GSM module
You can also test the module & commands by connecting it to PC com port & sending data serially using HYPER TERMINAL. https://www.dnatechindia.com/index.php/Tutorials/8051-Tutorial/HYPERTERMINAL.html