Google it!
Read as many sites as you can find.
Buy books (or look in your local library) on the subject.
Don't just look at one site, or read one book. Read everything you can get your hands on!
Read about composition and elements of design for fine art/painting/illustration/graphic design it applies to photography as well.
Read as much as you can, AND look for what you have read about in images from well known photographers (google them), in print ads, etc. Look for what you are reading about EVERYWHERE. Even TV and movies utilize the theory of composition. In video, you can watch transformations from one style of composition, the another.
Don't settle for reading one book, or one website. Authors will have different views, and describe things differently, so while one may be confusing, or lack specificity, another will give you that "ah hah!" moment.