The main compositional concept of placing your subject within the frame is called the “rule of thirds.”
Your subject does not have to be smack-dab in the middle of the photograph. Mentally divide what you see through the viewfinder into three rows across and three columns down, then compose your photograph so that the subject falls along one of the dividing lines.
In the first photograph, the seagull on the piling is centered within the frame. The red lines indicate where the rule of thirds lines are. This is a very acceptable photograph but not as compositionally pleasing as it could be.
The proper way to recompose the shot would be to move myself or move the subject. Since I could do neither, I used my zoom lens to zoom in closer to the subject and recomposed the photograph so that the subject fell on the rules of thirds lines.
In this photograph, two of the lines are used. The horizon is level with the upper horizontal line and the piling is along the right-hand vertical line, which results in the seagull being in the area where the two lines intersect. The intersection of two lines is called a “power point.” —CWN
FUN FACT
The term “rule of thirds” was used as early as 1797 as a rule for proportioning scenic paintings.