Hi.
Key question: What kind of hardware do you have/want to use/What is your background?
The easiest way for most people without prior knowledge would be Arduino with GSM shield, or a Raspberry Pi with a GSM module. The reason is due to the simplified programming (no need for assembly, writing library code etc), and the amount of documentation for these devices.
That being said, I suppose it deponds on your programming background, and if you are an electrician, if you want to
use ladder logic or programming languages like C/C++ and even assembly programming or to be able to easily use java, .net etc.etc
You can find step by step instructions for most common devices for the task u describe, and for the arduinos and raspberry pis, there is overwhelming amounts of tutorials and picture guides etc.
There is really no better friend for this question than google.