The UPSC conduct civil services exam in two different stages
1. Preliminary Exam
2. Mains Exam
You can choose your interested subject because in interview mostly ask related to your chosen subject.
I am also preparing for UPSC exam and when I have to need any preparation resource like books and coaching material, so i visit Prozo, where I get best study material for my preparation. I hope, This information will be helpful for your future.