A good composition can make a photo more pleasing and impactful. There are really no definitive rules that say that in order for a picture to look good, it must be like this or that.
However, there are some very general guidelines. One of which is the Rules of Thirds.
Basically this guideline states that if you were to divide a picture into 3 parts, both vertically and horizontally, the points of intersection should be where your subject be positioned in order for the picture to look interesting. See the picture below:
Under normal circumstances, our tendency is to put the subject at the centre of the frame. However, this usually produce a very conventional looking images which is not very interesting to look at. By applying the Rules of Thirds, a picture can become noticeable more appealing and interesting.
Beside this rule, there are several other rules that will lend enhancement to the composition as well. These includes the Leading Line rule.
Give it a try and see for yourself the result