There are two ways to enter into Criminal Investigation Department (CID) .

One is direct from the state police force through promotion. For this, the basic qualification is paas in 10+2 then complete your graduation, after graduation join police service then you will be posted/transferred or promoted to CID wing as per your track record or by seniority whichever is preferred by the seniors. To become a CID officer one should require to have at least two years as a uniformed officer. After that you can apply for the transfer to CID office post, there you will be trained. While training they will refer you as Trainee Constable. The training will be of 2 years, after training you will become a fully fledged Constable.

Other option is to give Indian Civil Services examination. The notification for the examination is published on the official website of UPSC which the candidates should read for any information. Kindly check their official website to get latest updates. UPSC examination comprises of written test, interview and physical test.

One must have detailed knowledge on current happenings in the society in order to succeed in interview. As, UPSC is one of the toughest entrance exam in India, you've to study a lot so,  stay focused and motivated and Keep studying !

Good luck for your bright future !