St. Thomas wrote the Summa in sections, and so he probably expect you to read it in sections too. Don’t overwhelm yourself
A lot of Thomist ethics is rooted in the Thomist natural philosophy and psychology, so it is wise to have a grasp of what St. Thomas means by the four causes,
teleos, nature, and soul, specifically the various powers of the soul.
You should just read Arisotle first, specifically the first two books of
Physics,
Nicomachean Ethics, and
De Anima. You could read them side by side with St. Thomas’ commentaries too:
dhspriory.org/thomas/
Christi pax.