The rule of thirds is the simplest rule of composition. All you do is take your frame and overlay a grid of nine equal sections. This means you split the vertical space into three parts and the horizontal space into three parts. Here’s what that looks like:
Generally you want to place important elements where the grid intersects. Here are a few examples:
The intersection points are where the eye tends to go first, so it’s useful to place your subject on one of those points. People generally tend to aim for the center, but that’s often less effective.
If you find the rule of thirds a little boring, try composing using Fibonacci’s ratio.