Actually You can do MBBS after class 12.
You have to have biology , Physics, chemistry knowledge.
You have to give NEET exam and have to get admitted in any medical college (govt or private) for your MBBS course.
Now MBBS is a 5 yrs course.
You would be taught everything which a normal general doctor should know.
You can not opt for any specialization in MBBS.
You have to study everything.
After MBBS if you want to become a sergeant you can give exams for MS.