The UPSC conducts exam every year for CBI, CID you can sit for exam such as SSC combined graduate exam. The exams are conducted by UPSC.
You must have passed 12th standard and secured good marks.
Age of candidate for cbi exam should be minimum 20 and maximum 27 years.
Height must be minimum 165 cm and chest 76 cms
If you pass the exam then you will be called for training. The syllabus is Principles of Criminology, Psychology of Crime, Criminal Law, Police Administration, Forensic Science and Medicine, Juvenile Delinquency, Correctional Management and Rehabilitation of Criminals etc. you can have all the detailed info at www.upsc.gov.in as well as www.cbi.nic.