Photography is a language. It's used to evoke feelings and communicate a message. Sometimes that message is loud and clear. Sometimes that message is, "Hey, I just got this camera a half hour ago and I'm pointing it at things willy-nilly."
Here are three mistakes that untrained photographers tend to make, and what you can do to avoid them.
Rookie mistake #1 - Always place the subject in the middle of the frame.
Not as interesting
You've probably heard of the rule of thirds - it's called a rule, but it's really just a guideline. It breaks up the piece into three horizontal sections and three vertical sections. The rule suggests that it's more appealing and visually interesting if you place the main subject along the axis of the left or right side of the piece.
Rookie mistake #2 - Taking photos with a tilted horizon line or background.
Unless there's a compelling reason to make that horizon line slant, your best bet is to keep it straight. And if it is tilted just a little, it looks like you weren't paying enough attention. There are times when it makes sense to deliberately tilt the horizon line. If you do chose to tilt the line, be bold. Let the viewer know you're doing it for a reason.
Rookie mistake #3 - Not getting close enough to your subjects
A great war photographer once said, "If your pictures aren't good enough, you're not close enough." Move in close. Only show the essential details. Leave out all the stuff that's not contributing to the picture. Think of your pictures as a person telling a story. Only include the essentials in your pictures, in the same way a good storyteller only includes the necessary, interesting details. And like a good storyteller, sometimes what you leave out of the picture is just as important and interesting as what you include.
Too much information, less interesting
Closer, more interesting