Design

There is nothing called the best subject, as it will differ from person to person's interest.
Find below the list of subjects that are taught during graphic designing:
1.Genetic skills
2.Typographic Design
3.Computer Studies
4.Basic Information
5.Color Theory from Computer
6.Graphic Principle and method o

Yes, M.Des from NID is the best option you can ever choose for yourself. The eligibility criteria are mentioned below:
http://admissions.nid.edu/NIDA2018/Mdes.aspx

Private sector jobs after Bachelor of Design (B.Des).
There are enormous opportunities for the B.Des graduates to secure a job in private sector. Many private organizations such as Boutiques, Hotels, Fashion Media, marketing sector, Furniture Manufacturing Units, and Quality Control Offices, etc offe

Yes, IIFA does have the interior designing course.

NID is the best institute in India for design courses.
For the that (Design Aptitude Test), there isn't much that you can do. Firstly and most importantly, you should have an aptitude for design. NID papers are designed in such a way that only those who have a creative side will be able to clear it.

There is no college which offers a degree course after 10th. It is advised that you complete 10+2 first.

Get yourself enrolled into either NID or NIFT for fashion designing. Prepare for the entrance exam and get through it.

Please fill the below-mentioned form to start with:
http://www.chasaidt.in/AdmissionForm.aspx

Now, that you have done your BFTech, you can intern in a fashion house or even work for a year, if you want to.
For postgraduation, you can do MFTech. That depends on where you want to do it. If you want to study in the US or the UK, then work in a fashion house and work on your portfolio. Remember,

Modern day photographer can work in many avatars, the notable among them include:
Press Photographers/photojournalists: Photojournalists supply pictures to the national and global press. This career role demands an ability to photograph all kinds of newsworthy people, places, sports, political, and c