Believe it or not, you can turn your camera on its side and it
will still work! So many beginners will not even think about the vertical
format and end up with a lot of empty space on the sides of the subject
that could have been eliminated by simply going with a vertical format.
Some subjects are obviously suited to a vertical format - anything that
is taller than it is wide. But, having said that, a tall subject could
also work in a horizontal format. Take a waterfall, for instance. If you
want to show the whole waterfall, fairly close in, use a vertical format.
But maybe you want to show the setting around it as well. So you back up
or use a wider angle lens and include some of the rest of the scene in the
image. (Remember what I said about rules being simply guidelines and not
to be strictly adhered to?).
Let's look at these two pictures. Same subject, same vantage point.
Both formats work. It depends on what you want to say.