Some things are in our control and others not. – Epictetus
The single best introduction to Stoicism by an ancient Stoic is the Enchiridion by Epictetus.
The word enchiridion is an adjective meaning “in the hand” or “ready to hand”. This is a short handbook of the summary of what Epictetus taught, compiled by his student Arrian.
He begins with what seems obvious, “Some things are in our control and others not.” But here is the key, we spend much of our time, attention, and worrying about things that are not in our control.
The Stoics taught that we need to “accept with serenity” the things we cannot control. If you can’t control it, then it is none of your business.
If you are just getting into Stoicism, the Enchiridion is the best place to start. To help you, here are two great resources to get you started for free.