now I am in 12th standard

I ask you only that what is the process of be a cinematographer?
how to be selected as cinematographer in travel shows on channels like discovery, nat geo

A cinematographers training never ends, primarily because technology is constantly changing and new techniques, as well as equipment, are developed. So, whether you are just starting out as a camera operator, or have climbed the ladder to director of photography, you must keep up-to-date with new gadgets and tools, and be ready to use them on the set.
At the certificate level, learn accelerated programs and work in labs to became familiar with filmmaking techniques. You may learn camera lens and lighting techniques, how to use exposure meters, the role of a cinematographer, and gain an understanding of film stocks.
A bachelors of science in digital cinematography (the preferred degree ) or else any graduation will work. After that take some internship and a certificate course in it.

Hope this guides you.