While you can read the Commedia without any extratextual study, as I did the first time around in 2003–04, it’s not really something that’s recommended if you want more complete understanding. Unless you’re coming to the poem with a pre-existing wealth of knowledge about Catholic theology, Classical Antiquity (history, myths, literature), Medieval Italian history, the Guelph vs. Ghibelline power struggle, and Dante’s own life, chances are you’ll miss out on a lot of things.
The most obvious place to start is a translation with lots of supplemental notes and essays. Many come with introductory summaries of each canto and footnotes, but don’t always have anything beyond that. Unfortunately, many editions only provide the basics when the entire Commedia is in one volume. You have to buy separate volumes of the three canticles for more in-depth notes.
Because I read the Commedia on my own instead of for a class, I was at the mercy of my translation, which provided only the aforementioned footnotes and canto summaries. Although I’m not sure I would’ve been emotionally and intellectually ready had I read it in my teens or early twenties. My lifelong advanced reading and love of classic world lit didn’t help me with epic poetry. That’s just not a literary form most modern people are used to, and teachers tend to throw students into the deep end with The Iliad and The Aeneid instead of starting off with something lighter and less dense.
Many English teachers also overanalyse everything and kill the joy of reading for pleasure. It’s hard to be genuinely moved by or fall in love with a book if you’re told what you’re supposed to think about it and how to react to it.
Though you don’t need to read and study all the books referenced or which influenced it (some more than others), it is helpful to at least have some familiarity with:
The Aeneid (Virgil)
The Metamorphoses (Ovid)
The works of Statius (Silvae, Thebaid, Achilleid)
The Confessions of St. Augustine
Summa Theologica (St. Thomas Aquinas)
The works of Aristotle, esp. Metaphysics
The works of Horace, esp. Ars Poetica and Odes
The works of Seneca
I’d also recommend reading La Vita Nuova before the Commedia, since it’s so much shorter and easier to understand, as well as providing a lot of useful background. It’s a shame it’s always put in the back in books which bundle Dante’s works together, instead of first. I understand why they do it, but it makes more sense to read La Vita Nuova first.
Other study sources you might find helpful:
Princeton Dante Project (full text of all works, plus lectures, maps, and more)
Digital Dante at Columbia (videos of lectures, commentaries, images, history, and more)
Canto per Canto (YouTube channel featuring conversations with Dante scholars, though they unfortunately don’t go in order)
Dante Notes (essays by students and scholars)
Dante Studies (free access available to members of the Dante Society)
Dante: The Poet, the Political Thinker, the Man, Barbara Reynolds, 2006
Dante’s Bones: How a Poet Invented Italy, Guy Raffa, 2020
Mark Vernon (YouTube channel with discussions of each canto, and other Dantean content)
Tom LA Books (YouTube channel with discussions of each canto, including helpful background information on many details)
Feel free to suggest any other helpful scholarly websites, journals, books, and vloggers!
Just as Dante needs Virgil to lead him out of the dark forest and start him on the journey of redemption, Beatrice to take him through the final leg, and St. Bernard of Clairvaux to lead him to the final goal, so too do we need teachers (whether professional Dante scholars or passionate laypeople with a wealth of knowledge) to increase our understanding. This isn’t a journey we should take alone.