Along with light, the other basic principle of photography is composition and framing. Photography literally means painting with light, so you want to paint with the light in such a way that fills the frame really well. I have a couple of examples here of photographs I picked that I think demonstrate that well, and I'll show them to you here.
I like to break my photos up into quadrants. I have various things going on in each quadrant just to keep the photograph balanced. This is another example. You want to avoid centering things. You want to have the frame filled, and just have something in all different sorts of places for your eye to look. This is sort of the way that I like to shoot. It does not follow the rule of thirds, but it's splitting the frame right down the middle, and sort of juxtaposing one scene to another scene. It's a really good trick. A lot of photographers do it. I think it's really effective. It's one of my favorite ways to shoot.
This is the final example, and this is a little bit different too. This doesn't follow the rule of thirds either. However, it fills the frame in an abstract way that the eye kind of just moves around. It's just another really good way to compose.
Every photographer shoots in a different way, and every photographer composes in a different way. Some people like to have leading lines that go to a single point off into the photo. That's another great way to shoot. It's not personally the way that I like to, but it's not back composition either.
One thing to remember when trying to avoid it is nine times out of ten, centering your objects or centering your subject is going to be bad. The photo, I've shown this photo before, this is a centered photo. The reason that this one works compositionally speaking is because the leading lines kind of bring you back to this point. So in this case it works, but in a lot of other cases it wouldn't.
Composition is really just something you learn as you experiment and as you shoot. As you get better, you'll see what works for you and what doesn't work for you. And different people have different thoughts and different rules about it that they like to live by. And it's just kind of a matter of personal preference.
These are just a couple of things to help you understand composition.