Where to begin
A first book, a first week, and what to skip.
The first hour
Read the Enchiridion. It is fifty three paragraphs and takes an hour. Everything else on this shelf assumes its first sentence, so read that first sentence until you can say it without looking.
The first week
Then read the Letters to Lucilius, one a day, in order, for a week. Seneca is the easiest of the three to read because he is the only one who was writing to be read. If Letter 1 does not hold you, stop and try On the Shortness of Life, which is the same argument compressed.
The first month
Now the Meditations. Start at Book 2, not Book 1, and read a book a week. Keep a note of the passages that repeat; Marcus repeats himself on purpose, and the repetitions are the map of what he found hardest.
After that
The Discourses if you want more Epictetus, which most readers of the Enchiridion do. Then the modern shelf, which this site will cover as pages are written.
What to skip
Anything with Stoic in the title and a business case in the subtitle. Anthologies of quotations, which give you the sentences without the arguments that earn them. Any edition of the Meditations that begins with a long introduction about the Roman Empire; you can read that afterwards.