How should I first study the Summa Theologica?

Do not let it intimidate you. Get the best English translation you can find and read it front to back. As you go, take notes, ideally in a book large enough to write in the margins, and do not be afraid to highlight, underline, and mark things up, because by the time you reach the end you will have forgotten what you read at the beginning unless you train your mind to hold it by bringing it repeatedly to your attention. Go over it again and again, so that when someone asks about question 62 you know roughly what Aquinas said. You have a great mind God gave you; put it to work and you can do far more than you think. The same principle applies to the Bible: people ask how to start studying it, and the answer is simply to read it, become totally familiar with the text itself before running to what others say about it, and let God's Spirit fill you as you read his words. You have tackled a big work in the Summa, so blessings to you, but the method is the same: go in, read it, and own it.