UPSC Prelims has 2 papers. Paper 1 covers Current Events, History of India and Indian National Movement, Indian and World Geography, Indian Polity and Governance, Economic and   Social Development, General Issues on Environmental ecology, General Science.

Paper 2 covers Comprehension, Interpersonal skills including communication skills, logical reasoning and analytical ability, decision making and problem solving, General Mental Ability and basic numeracy numbers. Now this is basic syllabus and if you can take up Geography, or History or Political Sciences in your 11th, it will be easier for you to clear Prelims. Because you can also select one of these again in Mains subjects, it will be easier for your preparation.