hey!!

if you are currently in 10+2 stage or less then :

for becoming  a doctor  in indian army you can apply in AFMC pune. 
Armed Forces Medical College(Afmc) is a premier medical institute of India acknowledged as a centre of excellence for education and research.
The college provides training to under-graduate and post-graduate medical and nursing students with assured career prospects in the defence services.
Armed Forces Medical College (AFMC), Pune will not conduct any entrance test for admissions to MBBS programme. Instead, AFMC MBBS admissions will be on the basis of candidates marks in National Eligibility cum Entrance Test (NEET) .
Candidates who will qualify NEET will be invited to appear in ToELR (Test of English Language, Comprehension, Logic and Reasoning), a psychological assessment test cum interview. 

or if currently doing mbbs :

After completing your MBBS from any other Medical College other than AFMC, you can join the Armed Forces Medical Services as a Doctor in the AMC in the rank of Captain(Army), Surgeon Lieutenant(Navy), or Flight Lieutenant(IAF). An interview would be held at New Delhi by a panel of Army Doctors and candidates are tested on their knowledge of medical subjects and attitude followed by medical examination of the candidate.
Army Medical Corps comprise IA, IN and IAF, hence you may get any of the three services on selection. Ratio of selection in Army to navy to airforce is usually 80:10:10 as army is quite huge compared to Navy or Airforce.
Selection usually takes place twice in a year - for January and June. So advertisement regarding selection is usually in months of August and February.
Other option is to do your MD/MS from AFMS institutes which come with a rider of compulsory 05 years of service as an SSC officer.

all the best