Web design

Dear Anujeet,
You have to complete class 12th if you want to pursue your Diploma from NID. However, you can explore other options like LISAA School of Design and The Instituto Marangoni.

Web design is the art of creating presentations (using HTML, DHTML etc) that is delivered to an end-user through the World Wide Web, using a Web browser. There are many institutes in India which offers Web designing courses. To know about these you may refer to the following link:
http://tinyurl.com/yjs7q7s

As per my view, it won't be a good option to quit electronics and opt for web designing. Better, you can join an institute offering web designing course on the daily basis and while pursuing your graduation. After graduation, you can look for external courses available in the field of web designing.

No, you don't need to appear for any entrance exam. You can directly take admission in an institute to pursue web designing course.

Software development is better as compared to web designing. Software designing is a bit challenging and dynamic as requirements of clients vary a lot. Most of the IT companies in India make money by taking development projects.

Yes, you have good chances to make a career in the advertising, account planning and media planning field with marketing houses and media agencies as well as with FMCG companies.

IT & ITES companies, IT department of business enterprises, small time web development companies hires web designers.

I would recommend you to follow the same order as the list below:
1. HTML and CSS.
2. Javascript.
3. JQuery.
4. AngularJS.
5. PHP and MySQL.
6. Ruby.
7. Ruby on Rails.