§ 01 · Introduction
Thomas wrote the Summa as a textbook for beginners in theology, which tells you something about thirteenth-century beginners. It is arranged in three parts: the First Part on God and creation, the Second Part (itself in two halves) on the human journey back to God through virtue, law, and grace, and the Third Part on Christ and the sacraments. Thomas died in 1274 with the Third Part unfinished; his secretary Reginald compiled the Supplement from earlier writings.
Every article follows the same shape, and once you know it the whole work opens up. A question is posed. Objections are stated as strongly as possible — Thomas is famously fair to the people he disagrees with. Then "On the contrary" gives an authority on the other side, "I answer that" gives his own position, and finally each objection is answered in turn. It is disputation frozen on the page.
Do not try to read the Summa front to back. Pick a question that matters to you — on happiness, on prayer, on whether God exists, on the virtues — and read that article whole, objections and replies included. The English Dominican translation is plain and exact.