Cinematographer

Hi,
First of all, you have to acquire knowledge in the cinematography field. The best way to start is to study Photography, since Cinematography and Photography share a lot of common elements. I would recommend the Series of Ansel Adans book's. Although they speak of analogue photography, his teachings can be applie for digital photography.
After you understand the basics of exposure and analog photography, you should advance in the studies of digital sensor technologies.
In the meantime, you should start reading books about cinematography.
There are three that I can't recommend enough:
The five C's of Cinematography (this is a more basic one)
Cinematography
Lighting for Film and Television
And you should also read the american cinematographer's manual
After that, you start to understand the theory of how to manipulate the light and to her elements that allow you to shape an image, you should start watching every film that you can grab your hands on (you should be doing that anyway) but you need to try to understand the technicalities of it (if you can find an American Cinematographer's article about a specific movie, that helps)
During all that, you should start taking pictures and learn how to manipulate then digitally. Take as much pictures as you can, but don't take it for granted, put effort in each shot.
You can also start shooting some footage. Again, put effort in it. Try to emulate scenes you saw somewhere. learn color correction.
All those things are going to give you a start on the theoretical knowledge factor, but you should also have piratical experience.
You must start shooting movies of your own. You need to find friends or a crew to work with. Maybe, you can join a film school perhaps.
You should also try to somehow start to work as an assistant on film crews, on the cinematography team.
You are going to hate the first few footage you produce, you are going to think that they do not look like what you see in movies (and they won't). But you will get better with time (but be warned, it takes time!).
But there's no easy way in, everything you do at first is going to feel like a small step in a infinite journey (and it is), and sometimes you are going to feel exhausted, and you are going to want to give up.
All the best.