Fashion Design

Depends on your college.
Better go for more reference and take further step in your career.
All the best

There's a Bachelor degree in fashion designing in some colleges. You can go for it.
Or
You can choose BBA or B.Com , according to me that degree don't have anything that supports fashion designing.
All the best

1. Vidya Institute of Fashion Technology
2. Axis Institute of Fashion Technology

Yes, you may pursue M.Des in Fashion Design. But, you will have to give entrance exams, to get into a good college.

Yes, you can appear for NIFT entrance exam after completing your 12th.

A lot of private courses are available on your requirement.However for a proper professional qualification would suggest you to pursue it through NIFT.
after 12th standard there is a open test for seeking admission in it.You can pick up your specialisation during the admission process.
There are other institutes as well who offer MBA in fashion technology/design ,however each institute has a open test that you need to qualify

Refer to the link below:
https://design.shiksha.com/fashion-textile-designing-courses-in-india-categorypage-13-69-1-0-0-1-1-2-none-1-0?

You can start education whenever you feel like starting it. You may apply for some certificate courses if you wish to and you can simultaneously carry on with your venture as well.
You may look for the courses in the below mentioned link:http://www.vskills.in/certification/Design

Medical and fashion designing are like north pole and south pole
It depends on your interest.how u visualize yourself in future
Medical is lot of hardwork and many years of study.its a long journey
If u have the nack for designing go for fashion
Do what u feel best and what interests you
Hope this helps
Good luck