Hi Monika,
Congratulations on completing your class 12 th.To appear for civil services exam you should first complete your graduation as it's mandatory.You can do graduation in any field of your choice.You can do B. tech, Mbbs, BSc,Bcom,BA,B arch, BSc Agri, BVSc , etc Choice of stream is endless.
After completing graduation you can appear for Civil Services Exam conducted by UPSC each year.
Hope this helps