To become a Lieutenant after B.Com you need to appear in the CDS (Common Defence Services) exam which is conducted by the union public service commission. It is conducted twice in a year in month of February and September by UPSC. Following are the details of CDS examination:

Eligibility for IMA:
The applicant should have completed graduation with good academic record from any recognized university of India.

Age Limit
19-24 years

Physical Standards:
Candidates must be physically fit according to physical standards for admission to Combined Defence Services Examination (I), 2014

Selection Process
Written exam (Objective type)
SSB Interview (Including PABT)
Medical Examination