Anyone can take a photograph; is it in fact fair to say that almost everyone considers themselves some kind of photographer these days, (especially when you take into consideration the capability of smart phones)? However, not everyone is a good photographer. Of course judging such things can mostly be put down to subjectivity, but there are well-established guidelines and practices - many of them very subtle - that turn an image from good to "Wow!" and see a shift from amateur to professional.
As a portrait photographer I am always looking out for ways in which to improve and so the lessons on how to crop an image are an ongoing education. Master the art of cropping and you take a key step towards better photographs. Below are some helpful tips from Digital Photography School on some of the basics - did you know it's best to avoid cropping on the joints of the body (the knees, elbows etc) - so have a read through and be sure to lift something useful from it.
http://digital-photography-school.com/good-crop-bad-crop-how-to-crop-portraits