The best way to learn is practice, practice & more practice. The most important thing is to enjoy what you are doing. There are commonly accepted "rules" about photo composition - typically the main subject should lie at one third the way across the photo, rather than dead-centre, but you don't always have to follow these rules. Correct exposure depends camera performance, and practice in different lighting conditions will show you how your camera behaves. And remember, photos in books, magazines & web sites may have been digitally adjusted ("photoshopped") to improve contrast and detail (or in some cases, to "remove" posts, wiring, etc., so that the photo looks "pretty" instead of being an "honest" view of an event.)