If you're interested in studying Thomas Aquinas, you should start with a simple biography. The biography by G.K. Chesterton is a fun read and I recommend it by way of introduction.
Next, you'll need a basic introduction to the history of Thomism. I recommend Romanus Cessario's A Short History of Thomism. It gives you the big picture so that you can wade through the debates that have occured inside and outside of Thomism:
Then you'll need a good English version of Thomas' Summa theologiae. I like the five volume edition depicted below. The translation closely follows the Latin:
If you're to be more extensive, you'll need Thomas' Summa contra gentiles, as well.
If you're into philosophy, you need to pick up Thomas' De ente et essentia:
That's enough reading, for now. If you're able to read Latin, go straight to the Corpus Thomisticum, which is online. You'll find everything you need there, and then some.
Click here to be redirected to the online Corpus Thomisticium.