Dear applicant to clear the prelims exam of UPSC there is are few compulsory subjects which require your utmost attention like History, Geography, Political Science, Sociology, General Science which includes Physics, Chemistry, Biology or Mathematics etc. But yes for general science you don't need years of practice of Science but only require your 10th knowledge for the betterment of the subjects. Now overall one thing to be made clear if you really wish to score well thn you should possess strong grip on the different concepts as well.

Good luck!!!