Generally, I approach this sort of problems step by step.

First, I read about preparing the SMS to memory, about AT commands, about hardware needed for interfacing.

Then, I build a cable and connect the device to PC, and try to send a command to send SMS from a terminal program from PC.

Then I write a simple firmware, e.g. "wait until keypress then send a single SMS" to the microcontroller. I am already familiar enough with serial communication on the microcontroller, as this is part of my job the last 15 years or so; but if this is not the case for you, then you might want to do the basic steps there as well - I can write it down in more details if you want.


Then I would write the rest of the firmware.

JW