MIT has a comprehensive course about lasers in three parts in here. Of course, the course might seem daunting but it goes into the depths of physics and optics to explain you how a laser works and how to obtain it.
Lasers are obtained when you get a chain reaction in an object like a ruby crystal or a gas. That chain reaction effectively emits photons of a given wavelength in all directions. Two mirrors will then reflect the light that is parrallel to the axis that connects them and the light will then escape from one of these mirrors as one of them has less than 100% reflectivity.
This way, when the light exits the mirror you get a focused beam of light that is monochromatic and powerful.
You see see short versions of how lasers work below:
– short explanation from Bill Hammack
– Einstein animation that explains how lasers work