There are many pathways into learning about Stoicism, but at some point most or all will find you reading through the Enchiridion of Epictetus.
Interestingly enough, The Handbook or Manual as many call it, was not written by the famous Stoic philosopher, but was compiled by one of his students.
It's a short, easy to consume introduction to Stoicism, composed as practical instructions to the reader.
It's worth noting that if you become very interested in Stoicism, you will probably find yourself reading from more than one translation of a work like the Enchiridion, and forming a preference to one or more translations over others. I personally prefer the translation by Robert Hard, but this isn't in the public domain. The linked translation is by Thomas Higginson.