If you think it seriously with photography and want your pictures to be more professional, you should pay more attention to the composition. And what does it mean? Composition is actually how the elements are arranged in the picture. Not only the main subject but also its surroundings. The reason why some picture compositions seems to be better than others is that the human eye perceives rather images with certain sense of order.
1. Among the most known rules belongs the king's Rule of thirds. And what is the theory? Your eye prefers images divided into thirds while the main subject is around one of those section. Some smartphones can show you a visual grid so you can practice first with this useful tool. If you cannot use this, just imagine the picture is divided into nine equal sections by four lines. A then just place the main subject at the right or left third of the frame rather than into the middle.
2. Other rule called the Golden ratio is more sophisticated. It divides the picture according to the lines concentrated in the middle of the frame. It could be stated preciesly mathematically but we think its better to see it visually to understand to this rule. The idea that a perfectly composed image should follow the lines in the rectangle has been known for centuries and you can find it in many masterpieces of great works of arts.