The rule of thirds is one of the most basic principles of composition in photography, and one you will use all the time going forward! Here's how to do it:
What you need to do is imagine that your photograph is divided into nine equal sections by two horizontal and two vertical lines (as shown in all the examples below) Apparently, when we look at something, our eyes are naturally drawn to the points where these lines intersect. Placing a point of interest on one of these intersections, or placing elements along one of the lines, gives the photo more balance and is more appealing to the viewer.
It's probably easier to show this with some examples. In this first image you can see that the little girls' eyes are along the top line, with one eye very close to an intersection. Additionally she is to one side, her body filling two thirds of the frame as opposed to being exactly in the centre.
In this second example, we are using exactly the same principle, by having the body along one of the lines, and eye on an intersection.
This one is from either farther back - you can see that the body has been placed along the right vertical line, and the horizon is along the bottom line. If you have an horizon in the image, try to get it to sit along either the top or the bottom line - the top if you want to show more of the foreground, or the bottom if - like in this image - the sky is more important.
And again, body along right vertical, eyes along the top horizontal line, table top along the bottom horizontal line, making the image "fit" to the rule in as many ways as possible!
If you look around at other images, you'll soon start to see that many images use this "rule" - in fact, other forms for art, such as paintings or even television programmes, will also use the rule of thirds (in fact, I can't watch a TV programme now without seeing how they place their subjects, both with regard to light and composition - I really need to get out more)
If you haven’t been using this rule up to now, it can sometimes be a good exercise to go through your old photos and see if they can be improved by applying the rule of thirds through cropping. Not only might it help with giving some photos a new dimension, it helps you to start to see how this works in practice.
Always bear in mind though, it’s a guide, and sometimes breaking the rule can result in stunning and unusual shots. This image below breaks the rules, but works better with the subject being in the centre - however, if you notice, the sea level falls roughly along the top line, and the sand break between the pebbles lands on the bottom line, so I've still considered the rule, even though I'm breaking it.
Laslty, don't force an image to use the rule of thirds if it just doesn't work. Here's another image where I have cropped it to fit the "rule" (image on left) but it looks very unbalanced, plus I'm making an awkward crop. It looks better not following this rule - and due to that it's not the most dynamic image I grant you - but it's better than when being forced to "fit" when it clearly doesn't.
For many people, using this composition just comes naturally, for others, it is something you can learn. When giving this a go try to do as much as possible in camera, perhaps leaving a little breathing room for cropping or straightening. You can do this by simply imagining this grid when you take the photo, as I do, or if you prefer, many cameras have the facility to turn on a display setting that shows the rule of thirds grid when you are taking the photo - so no more guesswork involved! You can also crop your photos using the rule of thirds grid overlay using Photohop, Lightroom or Photoshop Elements.
Above all remember that above all that the rule of thirds is a simply a guide - use it, and if it doesn't work don't be afraid to try something else instead! (And of course, composition is just one element of a successful image, there are many more! We cover them all in my e-course Auto to Awesome if you want to check it out)
If you want to take the first steps to getting better photos, then why not check out my FREE 7 Day email course for beginners, designed to get you taking better pictures in just a week!