There aren’t exact references to be honest. But, a few tested out ways are enlisted below for you to start with.
• Firstly, to start with, the best and a way that has been advised by most IAS’ and IAF’s during interviews is to, brush up your NCERT’s.
Pick up all the NCERT books from your school level (especially from grade VIII to XII ) and focus on reading and rereading them.
The subjects like geography, history, civics, economics, physics, chemistry, English, maths,… don’t leave any of the books of class VIII, IX &X.
From XI onwards, apart from history, civics, geography, economics, English (grammar) and maths, choose as per your extra or optional subject that you shall opt for during the UPSC entrance exam.
• Secondly, marry the newspaper ‘The Hindu’. That is, give it a read daily, without skipping a day.
Especially focus on the editorials and the perspectives, and that too especially the ones from the Thursday issue, every week.
• Thirdly, try to appear for different govt exams before you appear for UPSC, to experience the pattern of questions you shall have to face and get along well with the examination fever.
• Fourthly, if possible, take a short-termed ( 3 months or so) coaching for SSC, or any other exams that ranks lower than UPSC, to be in touch with a mentor. From then on, follow his / her advice, and start reading the books he / she advises you to.
• Nextly, if possible, join a test series. Appearing for the tests again and again would accelerate your preparation, believe me.
Do this, and then wait for a link I shall attach with the answer shortly.