What is the key to becoming a successful photographer?

I'm not sure how you define a successful photographer. The definition of success varies with the individual. So, I'll leave that up to you to decide what exactly success means to you.

As far as shooting for a major publication, I can give you some guidelines. First, you have to been proficient in the use of your gear. You have to understand light; how to see it and use it to produce the images you want, or are contracted to produce. You have to be able to "see" a photograph in your mind, to be able to visualize an image. You have to build a portfolio of images that illustrate how you see and capture the world around you. As all of this comes together, start looking for work as a photojournalist. Maybe, the school newspaper, the local paper, etc. This allows you to have a body of published work to build on.

Like any profession, photography requires knowledge, skill, and determination.

For starters, you would have to clarify your definition of "successful" photographer. Many people would see success as having a financial goal assigned to the end of that statement. However, if you managed to become a National Geographic Photographer I would have to give you props for that. National Geographic Photographers are some of the best in the world and they go to great lengths to capture those impressive images. If you accomplished that and could make a living shooting that would, in my opinion, make you a very successful photographer.
As for the second part of your question, I would be atrociously arrogant to even try to answer that question for you. I can offer you this. Study the publication and observe how its content and usage of images have changed over the decades. Determine how your work fits into the current NatGeo-framework and style of photojournalism. Also, know your craft! Photography has a lot more to do with individual passion and a desire to create than anything else, but the discipline to execute your vision with solid technical skills is liberating and essential. While you're doing all of that build a body of work and shoot, shoot and shoot some more. Make contact with the publication's Photo Editors and Senior Staff. The process will take time and effort. Stay with it and don't give up. Finally, ask yourself the one and only question that matters. How badly do I want it?
Fay

Photojournalism requires you to tell a story, sometimes with a single image. The image must be powerful enough to evoke emotion to almost anyone who looks at it. Start by reviewing images from magazines such as national geographic and then look around your local neighborhood and see what type of images that you can produce. A good photojournalist can find a story is almost anything so you need to have a good imagination. Of course you need to be on top of your game with the basics such a composition, lighting, focus that is as much as important as the subject that you are shooting. It can be an extremely competitive field that requires a tremendous amount of patience and yet a drive to get that one special image.

Geographic looks for journalists. My suggestion is to pursue a degree in journalism while honing your skills as a photographer outside the classroom.

Hey Gibson S.,
Good question. I'll start my answer by saying I have been fascinated with photographic images for more than 50 years, although I did not get into photography until I was around 20. Even today, I still look at the work of the great photographers and think about how they captured their images. You could say I am a student of photography as well as a photographer. Everything interests me because there is a great photo to be had almost everywhere I look. I started focusing on sports when I was finishing my degree at Boise State University and shooting for student media. I like shooting sports because there are so many opportunities for great images. And, I believe there is a fair amount of work available for sports shooters. Fortunately, I have a feel for sports and what's happening on the field so I can anticipate where the action will happen. That's something you can develop with time and practice. So, the easy answer is; I'm good at it and I figured I could make some money doing something I love to do. I tell people; "I am always practicing when I pick up a camera and I am always looking for the "greatest" image I have ever shot."

Do your own projects, consistently and passionately. Find the reasons for why you want to become a "successful photographer" and photograph it. The dedication and personal interest will show in the projects and you will be recognized as a photojournalist.