SIM800L is nice and inexpensive GSM breakout board. We will set it up with Arduino and send simple text messages. The library can ofcourse be used to do more things like calls etc., We will focus on setting the module the right way because you'll have to take care of a few things like power and reset.
Hookup
We will hook up the SIM800l module with Arduino using the Soft Serial library. We will connect the Hardware serial port to the computer to debug and print messages.
Reset Circuit
Power
Note that the module requires voltage in the range of 3.17 to 4.4v . If proper voltage is not provided the module will give under and over voltage warnings. Hence we used a 3.7v LiPo battery. You may also use a regulator like LM317.
Test Code
Let us start with the simple text code that will verify if everything is setup properly. Remember to insert a SIM card before proceeding further. The code below takes commands from the Arduino terminal and sends it to the GSM Module. It also sends the commands other way around, from the GSM module to the Arduino. The code will also verify if the Software Serial library is also working fine.