In photography, there is a well-known guideline for better composition called “The Rule of Thirds.” Simply put, draw an imaginary Tic-Tac-Toe grid on your screen. Then position your subject at one of the intersecting grids. You are essentially dividing the picture into nine equal parts by two equally spaced horizontal lines and two equally spaced vertical lines.
When you place important compositional elements along these lines or at their intersections, your photo will be more symmetrical and have more balance. This means the photographic subject will be more interesting than it would if you just placed it into the center of the frame.
The iPhone camera offers a grid to make this easier to pre-visualize as you make your photographs. To turn the grid on, go to settings → photos and camera → grid → toggle it on.
(iPhone 7+, iPhone 7, iPhone 6, or any newer version)