You need a GSM module, a microcontroller, and the AT Command set, oh, and a SIM card.
Sending "ATI<CR>" will normally respond with the modem model. AT means "Attention!", if you turn the result codes on then the modem sill sit until something happens, such as a voice call.
Sometimes the module will display a message with a "+" at the beginning, this is sometimes like an interrupt, where it comes in it will display it on the screen, other modules work in such a way that you have to poll the message engine, to determine if there are any new messages available.
Tis all in the AT command guide.
You have to make sure what level translation you need if any, e.g. RS232 requires the use of a chip such as the MAX232 to level shift RS232 to TTL.
TTL 3.3V may not work with 5V, so the tolerances etc will need to be checked.
RX goes to TX and vice-versa.
Just a few little tips!
Wilksey