Raiden

Short answer: Get a degree in photojournalism.

Then be prepared to pursue one of the most sought after photographic careers.

Make as many connections in the photography industry as humanly possible while your in school. You’ll need to use every contact and pull every string you can.

Be passionate about your photography.

Paul

Perfect your photographic skills. Work for a number of magazines shooting the kinds of assignments that are similar to those you see in National Geographic.

Once you have a body of work that compares favorably with the work you see in the National Geographic Magazine, they may be tracking your work and call you.

You can of course approach them with a lot of tearsheets showing your expertise shooting National Geographic style assignments and see what happens.

Aleah

first user said it, get into photography and journalism. studying wildlife and geography doesn’t hurt either.

Gerald

Here is a short profile of one of their staffers.
read it, check out his work then figure out what you need to do.