If you are going to use Aquinas, you should know your way around what he wrote. Here is a page where you can find most of what you need in English translation.
Here is how I’d describe it.
His Summa Theologiae is considered his main work. It is not the most detailed, but it is the most extensive. He quit before completing it, but he didn’t have a long way to go. It was finished by his pals. In some ways, the ST can be considered the main source on Aquinas: it is his most famous work. But if you want to get to the details, to write careful papers and figure something out better, you need to move beyond it.
The Summa Theologiae is laid out in a particular way. It has a first part, the first part of the second part, the second part of the second part, and a third part. So you have to understand that layout. You also have to understand that in each part there are sections that can be called whole treatises and can be consulted independently. He has a section on Christ and his work, a section on law, and so on, and each of these can be used as individual treatises.
Beyond that, each part is composed of questions, and each question is subdivided into articles (which are also questions—they are the larger question broken down into its component questions). Why does he ask questions? Because of his method. After you get the question addressed in the article, you then see this: various objections to the positive answer to the question, then a statement of the positive answer with some elaboration, which is the heart of the article, and then replies to the original objections.
So you get the title of the section which is the question—very important to read this if you are like me and usually skip titles. Then you have the objections leading off the article. These are the next-to-least important part, for all they come first. Then you have the heart of the matter, the answer to the question stated, then the explanation. Then you have the least important part last, the answers to the objections. All the parts can be useful, but you can make sense of what Aquinas is doing if you read the question first, then the response, then the explanation, then the objections, and then the answers to the specific objections. Or you can proceed in order, but I find it tedious and confusing to do so.
That is his method. Let me also say this about method. With method you get theology as science, and so you get systematic theology. Before Aquinas there was no systematic theology as such. If you look for method, you will not really find it; after him it becomes so foregrounded that eventually you get theological prolegomena, and beyond. When you start the ST, you will see him proceeding according to scientific method with two questions: (1) does it exist, and (2) what is it? Does God exist in order to be studied, and then, what is he. That is where he starts, and then proceeds to develop everything else from there. So much, then, for the ST.
His Summa Contra Gentiles is intended to provide theological answers to people dealing with unbelievers. It covers the same broad categories that the ST does, but it does so with a particular angle. Because it is not assumed that unbelievers will accept arguments based on Scripture’s authority, you will find arguments that avoid Scripture as much as possible. This is not because Aquinas thinks all doctrine can be rationally derived, it is because when you show up at a debate with an atheist or someone of another religion, you may not want to plan simply to preach a sermon.
The SCG is not laid out in the pattern of the ST. Instead, you just have the arguments for whatever he is affirming, and you don’t have the objections, response, and replies to objections. You can get straight to the issue, but in the ST you may get more detail. The SCG may, on the other hand, bring you into more detailed reasoning. And you can interpret one in light of another. If you click on one of the books that form the broad divisions of the SCG, you’ll get a list of the topics he deals with, and a sense of the order in which he proceeds. You can read his own statement of intention in the first book, and that is illuminating, and Scriptural.
An abbreviated version of both Summas is the Compendium Theologiae, which is classified in his Opuscula, his minor works. I think it is the place to start on any topic. This is the most concise statement of his theology, and from there you can move on to the broader statements in other works.
The opposite of the CT are his various Questiones Disputatae. In these disputed questions you have treatises which are the most detailed form of his theology, and if you want to deal with Aquinas seriously, you have to get into these. De spiritualibus creaturis, De Unione Verbi Incarnati, De veritate, De potentia – this is where it is at, the Aquinas of Aquinas, De anima, De malo, De virtutibus, Quodlibetales.
In all these works, keep clicking on the divisions until you get a list of propositions or questions, and this is what will give you a real sense of the scope of the work.
Aquinas of course commented Lombard’s Sentences. That is just how you started teaching theology in the 13th Century, and that work will illuminate his thinking in other works. Besides all these, you have the Opuscula, which include letters and sundry minor treatises, such as one on the eternity of the world. That one got him in trouble with the Platonists of the U. of Paris.
When doing systematic theology, you need several ingredients. You can find several of these ingredients unmixed in other of Aquinas’ works. One of the ingredients is obviously revelation. If you wonder how he arrives at some of his conclusions about particular texts, or are interested in a kind of glimpse of him doing his primary job (he was first of all a preacher, dedicated to preaching, and all his writing was intended to make the job of the preacher better), then you could look at his commentaries. He has commentaries on many of the books of the Bible.
Another crucial ingredient for Christian theology is philosophy. Good philosophy, that is. If you want to understand philosophy as Aquinas understood it, you can read his commentaries on Aristotle, which are numerous and weighty. This aspect of his thinking is crucial, of course, and anybody pretending to master Aquinas has to deal with his interaction with Aristotle. What is not so easy is that you have to understand the neoplatonic context in which this new appropriation of Aristotle arises. You’ll just have to study history for that.
You can find another useful translation of the ST here. It includes linked words you can click on to get definitions or even short treatises on, but not the parallel Latin. The layout of the various sections is stated in paragraphs, so it helps you also get a sense of the contents of each treatise within each division (1st, 1st of the 2nd, 2nd of the 2nd, and 3rd, remember).