Your paper is on the material dealing with the sin of the angels (or, better, the sin of the demons), discussed by St. Thomas explicitly in Summa Theologiae 1, q. 63 and q. 64.  As always, the questions leading up to these should also be read.  St. Thomas also discusses some of the issues taken up in q. 63 and q. 64 in the following places available on line in English translation:  

De Malo, q. 16, a. 3
Summa Contra Gentiles 3, 107-110
De Veritate, q. 24, a. 7 and q. 24, a. 10, and q. 29, a. 8

Your task is to explain in a well-thought-out and coherent way the main conclusions that St. Thomas reaches on the following topics:  (a) the possibility of sin among the angels; (b) the nature of the sin of the angels, (c) the timing of the sin of the angels, and (d) the impossibility of repentance for the angels.  If you want to address one or two other problems, you are free to do so but not obliged to do so.  Just stay within the space limits.