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 of design Introduction to Multimedia
7.Digital Publishing
8.Visual Communication
9.Sound Principles
10.Introduction of computer animation
11.Modelling using CAD
12.Multimedia Authoring
13.Digital Portfolio Development & presentation
14.Web Design
15.Flash & Scripting for Web
16.Principles of management
17.Web campaign Implementation
18.Drawing as Basis for 2D and 3D Animation
19.Design: Character
20.Background & Concept
21.Animation the production process
22.Composing/shooting on film
23.Sound recording breakdown: Voice
24.Music & Effects film
25.Language career planning and guidance
26.Team Management

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 offer huge openings for the B.Des pass outs. In these private firms, one can join as cutting assistant/graphic designer/ weaving consultants/ illustrator/ fashion merchandisers/costume designers/textile designers/ fashion forecasters / entrepreneurs / fashion stylists / design manager / colorists / outside sales representative.
Government sector jobs after Bachelor of Design (B.Des)
Besides the private sector firms, numerous government organizations also provide vast openings for the B.Des candidates. The government institutes such as National Institute of Fashion Technology (NIFT), National Institute of Design, The Indian Textile Industry, etc., recruits talented B.Des graduates for various posts. One can also look out for a profession in any of the nationalized banks or railway or defence services. To acquire a career in these government departments, the candidates need to clear the exam organized by different governmental organizations such as SSC, PSC, UPSC, etc. One can also choose teaching field which is considered as one of the noblest professions.
Job Prospects Abroad after Bachelor of Design (B.Des)
In addition to the variety of jobs offered in India, the candidates can also search out for a profession in various foreign countries. Exhibitions and auctions of ones creative works can be conducted in various parts of the world. Workshops can also be conducted. Various industries in different sectors abroad such as audiovisual media, design & digital arts, industrial design, interior design, interior architecture, product design, multimedia design, visual communications design, and so on recruit both freshers and experienced ones who have completed their B.Des course. One of the key benefits of securing a career abroad is that the remunerations offered is much better than that in India.

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. The question paper is a mix of creativity, logical thinking and general knowledge. You need to be aware of the latest happenings in the world. Cramming up yearbooks won't help you, seriously. Instead, make a habit of reading the newspaper every day.
After clearing that, you'll have to prepare for the studio test and the personal interview. The studio test judges you on your material handling and of course creativity. You could be provided with any material, like clay, terracotta, cardboard anything and from a set of themes, you may have to design something of your choice. Since it is time-bound, you have to be real quick and efficiently use the materials provided to you.
The personal interview is where the faculty evaluates you in person. They may ask you about your hobbies, interests and even talent. They may even ask you to sing if you're a singer. If you have a history of artwork, you get to showcase it to them. Whatever you have done till now, be it a random sketch or doodle, bring it along. But make sure it's your own work otherwise you may get caught and it's not that difficult for them to do so. Make sure that your portfolio stands out. It should speak for you and define you. Don't try to include or steal somebody else's ideas. Be unique and come up with something from your side. Follow your instinct.
For all this, you can join some coaching institute which will help you enhance your creative side. There are a lot of coaching institutes like BRDS, Skyblue, Silica, Pahal Design and the list goes on. But I would suggest you go ahead with BRDS because they have the maximum selections every year and also the material provided by them is good. Even if you don't wish to join any coaching institute, that's okay. I have friends who have cleared NID without joining any coaching and who even after joining such institutes haven't cleared any of the design entrances.
Lastly, don't stress about the exam much. Stress makes you think less creatively and that is something you wouldn't want to happen on the day of the exam. For the interview, don't worry at all. Be confident and be yourself.
All the best.