Rule of Thirds.
Here we are with the number three again. Again, this is incredibly important. For the rule of thirds, imagine you divide a picture into grids of equal size 3×3, four grid intersections within the box. The 4 points of intersections indicate the photographer what areas are the most appropriate to place the objects you want to be the center of interest. In case there are two points of interest, you would try to place the subjects on opposite sides.