Crime Investigation Department is a state Subject. Mainly the State Government assigns its team of CID member mainly from the IPS & IAS cadres. Now getting into the CID isn't that kind of direct process that you can join it by qualifying directly. At first you have to join the IPS or state Police service through the UPSC CSE or State Civil Service examination. After that, with some experience and on selections of internal assesments in the department you can get into CID. But directly there are no such things.

So try to target the Police service cadre in your Civil Service examination.

For more information regarding UPSC CSE consider the link below:

https://competition.careers360.com/articles/how-prepare-for-upsc-ias

I hope this helps. Wish you a great eve!