Hello,
There are many ways one can get into Intelligence Bureau.
You need to appear for SSC CGL examination and choose IB as your preferred service.
Or to join as higher level officers you need to clear UPSC Civil Services Examination, then at later stage of your career you can be called for deputation under IB.
IPS officers are also called to work under IB for a specific period of time.
However, SSC CGL examination is the best direct option to join intelligence bureau.

Follow this link for more information on SSC CGL examination
https://www.google.com/amp/s/competition.careers360.com/articles/ssc-cgl/amp

Good luck