Question: How to join CBI ?
Ans:

 To join CBI(Central Bureau Of Investigation ) you have CGPE( Combined Graduate 

Preliminary Exam) which comes under UPSC SSC ( Staff Selection Commission ) Exam.

Eligibility Criteria :
Graduation with minimum 55 %
age: 20years to 25 years.

for more detail visit 
http://www.cbi.gov.in/

http://ssc.nic.in/SSC.html