Rule of Thirds
When most people first start getting into photography they tend to center the subject smack in the middle of the frame. Although once in a while this works, usually it does not look right, or at least not all that good. In art classes they often will talk about the rule of thirds, and this is even more important in photography than in other art forms (at least IMHO). The two main rules I always teach my students right away in beginning classes are to fill the frame and rule of thirds. As long as you can get a good exposure then these two techniques will do more to transform your photos easily than can anything else. And they both apply to virtually all the photos you will ever take. Not all photos will use negative space, framing, or leading lines. But you will see most all good photographs using rule of thirds and filling the frame.