One of the most common causes of a blurry photo is motion blur. You simply move when taking a picture or your subject moves so that the picture is out of focus. Another cause is the slow shutter speed. The movement between the subject and the speed of the shutter does not match, so by the time the shutter closes the subject has already moved making the picture blurry. One other factor that you should consider is how you hold the camera.
A firm grip is necessary to avoid shaking which can produce a blurry image. Other factors that you must take care of is the focal length of the lens as well as the internal vibrations of the camera, which can blur the picture, without any external exertions.