Expression is letting your thoughts or feelings be known.
Most of us use words to do this. As a photographer you’ve chosen to use a photograph.
Unless you’re working as a photojournalist, you can use your camera as a means of expression. Instead of words, you can let your thoughts, feelings and emotions be known by making a photograph.
In order to express emotion you need to feel emotion and vice versa. It’s a continuos loop.
Next time you’re photographing something instead of simply photographing what your subject LOOKS like, photograph how you FEEL about what you’re photographing.
Once you examine how you feel, try to express your thoughts and feelings with your camera by using your aperture, shutter, speed, ISO and all of the other elements of photographic composition to visually communicate what your thoughts and feeling are about your subject.
These requires a good understanding of your camera, a good understanding of the basics of photography and an ability to step outside of yourself and examine your thoughts and feelings about whatever it is that you are photographing.