First install an application "Tera-Term" or X-CTU or Hyperterminal
Connect you GSM Module to PC
Download datasheet of Sim900 "AT Command" there you can get all the list of AT Command
But for simplicity
first type "AT" follwed by Carriage Return and Line feed , In the terminal just press enter after "AT" you will get OK as response
Same way for calling insert a GSM Sim card
and type "AT+COPS?" press Enter you will get the response regarding the operator name
For Call ATDphone_number; and ENTER ;// Call will be made
Code:
AT+CMGF=1 // Configures as Text Mode
AT+CMGS="phone_number"; ENter
then you will get the response like this
">" then type your message and after that
press
"ctrl+Z" // This will send the message to the number you entered
SO these are the primary ones try more and the same thing with microcontroller
Good luck