Hello Manish

Those days are gone when National Geographic used to hire full time photographers working for their them.Nowadays, most of the photos you see in their magazines are from freelance photographers. Even if they hire anyone for a project, those are contract based, already well known and well settled photographers with decades of experience, who'll have to pay for their own travel expenses, food, equipments, lodging, insurance etc, and then will be paid in accordance to their footage/photographs.

Basically, National Geographic doesn't hire first timers.

So, keep working, get your work published in magazines and papers, display your work in exhibitions, and when you think you are qualified enough to be a Nat Geo photographer, apply.

Also check their website once and send your applications to Professional photographers maybe they can keep you as an intern.

hope this helps you