You have to do your graduation first atleast after that you can apply for RBI grade B.
The eligibility for the RBI grade B exams are the Following
Officers in Grade 'B' (DR) - (General)
At least 60% marks in aggregate for all semester/ years (50% for SC/ ST/ PWD) or an equivalent grade in Bachelors degree as well as in 12th (or Diploma or equivalent) and 10th Standard examinations from recognized University and Boards respectively.
Officers in Grade 'B' (DR) DEPR
A Master's Degree in Economics/ Econometrics/ Quantitative Economics/ Mathematical Economics/ Integrated Economics Course/ Finance, with a minimum of 55% marks or an equivalent grade. SC/ST/PWD candidates are allowed a relaxation of 5%

Officers in Grade 'B' (DR) - DSIM

Essential: A Master's Degree in Statistics/ Mathematical Economics/ Mathematical Statistics/ Statistics & Informatics/ Econometrics from IIT-Kharagpur or Applied Statistics & Informatics from IIT-Bombay with at least 55% marks in aggregate of all semester/ years or an equivalent grade OR

Master's Degree in Mathematics with at least 55% marks in aggregate of all semester/years and 1-year PG Diploma in Statistics or related subjects from reputed instituteOR

M. Stat. from Indian Statistical Institute with a minimum of 55% marks in aggregate of all semesters/years OR

PG Diploma in Business Analytics (PGDBA) jointly offered by ISI Kolkata, IIT Kharagpur and IIM Calcutta with at least 55% marks in aggregate of all semesters/years or equivalent grade.

Note:ForSC/ST/PWD candidates, the minimum marks required in Master's Degree above is 50% in aggregate of all semesters/ years or an equivalent grade.