The rule of thirds sets the ground for balanced, interesting pictures. I love breaking rules, but I do think that it is important to learn them before doing so, after all- rules are there for a reason.
Divide the image into thirds using two horizontal and two vertical lines. By placing subjects in the intersections and the lines your photo will be more balanced and offer more natural viewing.
It is important to ask yourself what is of interest when deciding on the composition is; what do you want the viewer to focus on?
Once you practice this it will quickly become second nature. Then it is time to experiment with breaking the rule.