You can start your preparation by reading newspapers on a daily basis. Also, you can revise Class 9 and 10 NCERT Books (Social Studies, Science and Economics). Moreover, if you opt for Arts Stream that will be advantageous for you a questions from Arts subjects (like History, Geography, Economics) will be asked in Prelims (GS) section of UPSC. Hence after Class 10, all you need to do is develop newspaper reading habit and make your basics (of subjects taught in earlier classes) strong.

Also, it is important that you know the syllabus of UPSC. You can check it out from this link - UPSC IAS Syllabus 2022 - Paper wise PDF for Prelims & Mains Exam (careers360.com)